How to Re-Wire Your Car (Replacing the Wiring Harness): Part 2

Hopefully you’ve read Part 1 of this article, and also watched the video I’ve published talking about this. If not, I recommend grabbing a mug of tea and reading / watching those first – this will make a lot more sense. In part 1, I talked about labelling the new wiring harness whilst it’s out the car, and whilst you still have the old harness in your car to refer back to. Whilst time-consuming, this made the following steps much quicker and less daunting.

So now you’re ready to remove the old wiring harness.

Step 2: Disconnect the Old Harness

Firstly, disconnect the battery Negative and Positive terminals in the boot, and remove the battery just to be sure no wires touch it. Throughout this process I recommend trying to keep the old harness in one piece as far as possible – there may be wires, connectors, bulb holders or cables that you may need to re-use in some way when the new harness goes back in. And it may be that you still want to refer to the old harness in later steps.

I also recommend that wherever the old loom is connecting to other wires or equipment, before disconnecting the wires make sure you can tell the terminal numbers or the colour-codes of the other wires, otherwise you’ll get into a mess here. On my car, the wires that come out of the wiper motor were impossible to identify, so I cut the main harness to leave ‘stubs’ of identifiable wire in the bullet connector – at least that way I knew what wires go where when re-connecting the new harness. Obviously this breaks the notion of keeping the old harness as complete as possible, but it was the safest way to do this. Alternatively, label the cables or terminals, but be sure the labels don’t fall-off!

Now start disconnecting the old harness in the engine bay. Work logically and take lots of photos of each terminal or component as a record. Disconnect the lights in the wings – there are bullet connectors at the front right and left of the engine bay. Disconnect the wires from the fusebox and the surrounding components. Also don’t forget to disconnect the reverse light switch and starter inhibitor switch cabling that run to the gearbox. These come through the bulkhead just to the left of the heater matrix. You’ll also find the starter relay to the left of the heater matrix.

In total there are 4 places where cables come through the bulkhead into the engine bay:

  1. Left side of bulkhead, behind left bonnet hinge.
  2. Just to the left of the heater matrix.
  3. Left of the fusebox.
  4. Right side of bulkhead, behind right bonnet hinge.

Once done, move inside the car. You’ll need to remove the cardboard cover under the dashboard, and also undo the front of the main instrument binnacle. Once unscrewed, this can then be pulled backwards slightly. You’ll need to get your hand in there to disconnect the Speedometer and the trip reset cable – this is easier if you first disconnect the other end of the trip reset cable from the dash rail. The binnacle front should then move back a bit further. It’s very cramped in there.

Start disconnecting the wires from the instruments, switches and warning / illumination lights, taking lots of photos. Once every wire is free, the binnacle front can be removed, This is a great time to recondition the binnacle or the instruments themselves. On a Coupe don’t forget the instruments slung inner the main binnacle – these screw into the main binnacle from underneath, and some have a short extension harnesses for lamps – keep this, as it probably won’t be part of the new harness.

Next, move to the left front footwell. Remove the trim panel in the footwell (it pulls away), as behind here is the connection point where the front harness connects to the rear harness. You’ll should see a bunch of bullet connectors pushed into the cavity on the left – disconnect these, taking lots of photos first.

Finally, there’s the centre console. You’ll need to remove this, as the cabling passes through metal frame. Remove the switch panel, disconnect the wires, and don’t forget the gear selector illumination cabling as well. Also there are normally cables under the dash (cold-start warning light switch).

Step 3: Remove the Old Harness

OK, so you should now have the harness disconnected. The next challenge is removing it from the car. To do this you really need to remove the bonnet hinges – the harness snakes around the hinges after coming through the bulkhead into the engine bay, and it’s impossible to remove the old harness (or fit the new one) with the hinges in place.

Once the bonnet is off, push/pull the harness back through the bulkhead into the car. Try to preserve any grommets that are in good condition – the original Lucas grommets are very soft and easy to fit, but are almost impossible to find. Modern cheap grommets are horrible in comparison. Once you have the harness inside the car, work from one side to the other threading the old harness out.

Now is a great time to recondition the dashboard coverings etc. The car will be looking pretty awful, but things will get better! As mentioned, try to preserve as much of the old harness as possible.

Step 4: Fit the New Harness
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Just do everything you’ve just done, but in reverse, Simple! Well, actually, no, but this step needs some patience and strength to get the new harness installed correctly. Start by placing the new harness across the dashboard, laying parts of the harness in approximately the correct place. Now, you need to get the harness ends back through the bulkhead into the engine compartment. You need to do this without damaging your expensive new harness, but that’s not so easy when you have small, sharp-edged holes to get through.

My solution was to buy a roll of thick polythene tubing, they type that can be used by companies to create heat-sealed bags. This is ideal to protect the harness as you pass it through the bulkhead – the polythene may get damaged, but your harness should be safe. Wrap the harness in the polythene, securing it tightly with PVC table, and start pushing/pulling the harness through the bulkhead. It should go through eventually. Do this at all 4 places the harness passes through the bulkhead, positioning the grommets correctly – these are absolutely necessary otherwise the panel will cut into the harness.

Use the same trick wherever the harness passes through a panel.

I used the same polythene tubing to pass the harness through the passenger footwell, and also through the metal frame of the centre console. Once the harness is in place, carefully unwrap the tubing and pull it off the harness, being careful not to let the panel edge damage the harness. As you do each one, refit the harness grommets. Once they’re fitted, you’re past the worst.

Now is a good time to re-fit the bonnet and hinges, making sure to route the harness neatly around the hinges, and replace the plastic edge-trim that prevents the hinge cutting-into the harness.

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Step 5: Connecting the New Harness

Now you need to connect the new harness. This part needs time and patience, as mistakes now will take much longer to find and fix later.

Take your wiring diagram (see the Shop!) and a dry-marker pen. Work through every cable and component, checking the terminal numbers, wiring colour etc, and connecting it up. Make sure wires are routed as neatly as possible.

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Make a final check that all wires of the new harness are connected, and it all looks correct.

Step 6: Testing the New Harness

This is the scary step, but don’t panic – follow these steps and you should be fine. Firstly, remove all the fuses from the fusebox. This means that even with the battery connected, most of the harness will be ‘dead’.

Now make-up a short cable with an inline fuse holder, and temporarily connect that to the positive battery lead. I’d suggest starting with a 2Amp fuse. This is a  safeguard in case something’s wrong when you reconnect the battery.

Now, connect your fused lead to the battery, and then connect the earth lead. You shouldn’t see,  hear or smell anything bad, and the fuse shouldn’t blow. If you removed all the fuses, nothing should be visibly powered at this point – no lights on, the clock shouldn’t be working etc. There is power in the harness, but only the bare minimum.

The next step is to add each fuse in-turn, starting at the top, and test each circuit. Measure some voltages making sure they’re as expected. To make this easier I split my wiring diagram by function, so you can just look at each part in turn. Once happy with each circuit, remove that fuse and insert the next fuse.

If a fuse blows when inserted, you know that that circuit has a short, The only thing to do is trace that circuit, checking all connections carefully. Hopefully, if nothing else was changed, you’ll find the issue. I recommend that you don’t disturb anything else through this process such as the wiper motor, as that adds another variable – at least of you haven’t touched these things you’ll know it must be the harness that’s wrong. I didn’t follow my own advise here, but fortunately it worked out OK for me.

Once you’ve gone through each fuse and are happy everything works as expected, start adding the fuses again, but leave the previous fuse in-place. Again, check that everything works as expected. At the end of this, you should have a fully-working car again.


Throughout this process, it’s particularly important to work methodically and carefully. One incorrect wire can disable the whole car, so care is definitely needed. But by putting effort into Step 1, you can hopefully prevent any problems later. I was fortunate in that apart from a couple of things that needed terminals cleaning, everything worked after reconnecting the new harness. It can certainly seem daunting, but if I can do it, you can do it!

In the third-and-final part, I’ll discuss some of the changes I made to my new harness, mainly to make the car safer to use. By making these changes to the new harness before fitting it, I could re-wrap the harness so that the changes look completely original.

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5 thoughts on “How to Re-Wire Your Car (Replacing the Wiring Harness): Part 2

  1. Hi Richard, I have to do some electrical work on my P5, and was just looking to see what tools you used for the bullet connectors. I noticed that the link for the crimp tool is cancelled. I have looked around for that tool but it £88 plus VAT. If you have finished your work would be interested in selling it on to another rover owner.
    Kind Regards, Tony.

    1. Hi Antony, I hadn’t realised that ratchet crimp tool had become so unavailable and/or expensive. I just did some searching and i’d guess the manufacturer has stopped producing them. I’d prefer to keep my tool as I still plan on tidying the wiring at the back of the car,

      It looks like there’s a couple of options – there’s a simple tool that’s widely available that just seems to squash the bullet on 2 sides – it probably works OK-ish but wouldn’t be as good as a hex crimp, as the contact areas would be smaller between cable and crimp, and also moisture could enter the crimp quite easily I would think. It looks very expensive for what it does.

      If I were you, I’d look at Autosparks website – they do a ratchet hex tool that creates a hex crimp, and it’s a lot less expensive than the other tools (about £50 inc. VAT currently). Not cheap, but half the price of other tools. I’ve always found Autosparks to be a quality company and they sell items that just aren’t available elsewhere. The tool is here:

      If you do buy this, let me know what you think of the tool, as its quite a problem if we need to pay £100 just to crimp a bullet connector.

  2. Hi Richard,
    I trust all’s well and hope that your car is back on the road again.
    Thanks for posting this fascinating on the rewiring. So far I’ve steered clear of the electrics in my car but due to my mechanical fuel pump leaking, I have to fit an electric pump.
    I’m OK on the pump installation but need to fit a relay for it. There are many videos on YouTube about relays but of course none for the P5b. I’ve never fitted a relay before and I would be really grateful for your advice on where and how wire it up.
    Many thanks you in advance for your help.

    1. Hi Andrew, sorry for the slow reply – life getting in the way of things! I’d recommend fitting an inertia cut-off switch with an electric fuel pump. Modern cars have these, and they should avoid petrol being pumped over a hot engine in the event of an accident. I saw the discussion on the Forum about whether a relay is needed – it depends how much current the inertia cut-off switch can handle, and John knows what he’s talking about. I just went with a relay as a default. Give me a day-or-so and I’ll post the circuit diagram for the whole thing on here – I’ll make it an article.

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