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Boot Light and Rear Brake light issue


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UK '06 plate MK2

So this has been ongoing for a couple of years now. At first the center brake light stopped working and started blowing fuses (just the brake light one). Some investigation with a multimeter found a phnatom 12v on what should have been its ground wire (if I recall correctly). Hence apply the brakes = short circuit = blown fuse. It was only the centre one that did this so my workaround was to install a bypass wire from one of the side brake lights to it and tape up the dodgy wires.

A year and a half later of smooth running and the fuse went again. And it confirmed an old theory I had - if the boot isn't closed properly (inc warning light on dashboard, which works) and you use the brakes the fuse blows. Another thing I should mention is that my boot light hasn't worked for ages too, but because it's small and pointless I'd ignored it and not bothered replacing the bulb. Well it turns out it's not the bulb, it too seems to have a phantom 12v on it, whether or not the boot is open or closed. 

Today I've taken the boot latch apart and am satisfied the microswitch in there is fine (as noted, the door-is-open warning light works). Started removing paneling, can NOT find any shorts in the cabling between the hatch / gaiter all the way down to the plug behind the left bulb cluster. I can imagine that water COULD get in there, run down the cable and start shorting things in that plug but it is dry as a bone. And the boot light doesn't plug in there anyway, its wires snake off down the right side of the car.

So I'm starting to think the problem is somewhere near the front, don't suppose anyone has any clues to give me before I literally gut the thing trying to track the mystery 12v down!? Some pinouts of plugs (boot harness and rear latch particularly) saying what voltages should be where would be helpful!


boot1.jpg = general view of me pulling wires out, you can see the bypass wire and the old bad one hanging around with tape on in the top right

boot2.jpg = multimeter reading showing 12v on one leg of the bulb (which is good and isn't lit so it must be a very low current) , this is same if latch is closed or open


boot 1.jpg


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This is going to difficult to explain, but lets try anyhow !!

I can't see where you have the positive lead of your multimeter connected in the photo but I'm going to guess the metal bodywork, so its connected to chassis ground (negative). Lets hope I guessed right.

So looking at your picture I can see the voltage is -12.26 which would be correct because your meter leads are crossed over (Thats not a problem). What it does tell me is that there is no ground/negative connection to that bulb otherwise it would be lit up. The brown wire should be the ground and the blue/black wire is the positive.

You need to test for an open circuit ground/negative wire.

Remove the bulb from the fitting and use your meter on the resistance range. Measure the resistance between the brown wire of the fitting (No bulb fitted) and a good solid earth point on the body. You should get a value of less than 2 ohms.

Do the check and let me know what value you get and I will see if I can help you trace the fault.

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Thanks for offering to help. The reading I got was 800k, I think. the bulb has two wires - black and blue and orange and blue. I'm assuming you meant the latter

edit: also I confirmed it's not a problem with the harness, unplugged it and did a whole load of continuity tests and seems fine. Interestingly the blue and black wire going into the connector has continuity with the blue and black wire on the bulb on the "car side" of the plug, but on the "boot side" this wire is the center brake light wire that I have disconnected!



Edited by mrsuffolk
added extra detail and image
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Whilst the top centre brake light is desirable for safety, in case anyone is not aware, the mot manual says the tester must not fail it is the light has been disconnected, and says if there is any doubt then the tester should assume it has been disconnected.  I would want mine working but I thought I would mention this in case you thought it’s a mot problem

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Further investigations suggest that the Orange+Blue wire should actually be floating at 12v and the lamp should come on when the boot switch is open, connecting it to ground. The boot-is-open warning light appears to light up when the respective connection (C99) is NOT grounded.


EDIT: NO THIS IS WRONG >>> I think the same might be the case for the brake lights too, but I'm still looking through the diagrams.



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I think both issues are connected. Literally. Looking at the wiring diagrams it appears that on the boot connector (inset photo in boot6.jpg) that the black+blue wire and the green+yellow wire are swapped. Black+Blue is the "Luggage Compartment Lamp" (see previous posts) and Green+Yellow is the "Center High Stop Light" ... like how has this ever worked?! 

Will do some more testing in the daylight hours but I've had this car nearly 5 years. It's 15 years old. Has every owner just been replacing brake fuses every so often!?



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4 hours ago, mrsuffolk said:

confirmed an old theory I had - if the boot isn't closed properly (inc warning light on dashboard, which works) and you use the brakes the fuse blows.

I would agree looking at the photo of the connector the 2 wires are crossed. That also explains the thing with the boot open and brakes on blows the fuse as the brake light circuit is grounded via the boot latch switch for the boot light.

It would seem the car has always had a problem, and just nobody has (a) noticed (b) cared (c) looked in to it.

Swapping the wires over (and removing your temp wire) should sort the problem. Keep us informed of how it goes.

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yeah I've been pouring over the wiring diagram and the warning-light = fuse blown clinches it for me, that would connect the +12V that is supposed to turn the brake lights on directly to ground via the boot switch. Will be doing more testing in the morning before I go ahead and swap the wires at any rate. What I can't figure out is how it ever worked I'm certain that the boot light used to work - the whole time I've owned the car the boot light has been down the back of the wheel well and I could see the light poking out from the edges of the liner - and I've spent some time the past few days doing odd jobs to the car, one of which was fishing out the light. My guess is that the boot light must have been draining via the center brake light and slightly (but invisible to the naked eye during daylight) illuminating it in the process - until I did my bypass bodge.

Thanks for your help!

Also every car should just come with a 150 page PDF full of wiring diagrams, this thing has been a godsend! 

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Swap done, properly crimped, gaffer tape is just to help contain it and protect it from bumps and scrapes. Boot light comes on when boot is open, turns off when boot is closed. Brake light works with boot open without blowing fuse (pan lid test technique patent pending). What a ride that was! 

Thus I come to the end of my summer odd jobs list:

- Boot light found and fixed
- Ongoing brake lights issue fixed (bonus item!)
- New washer pump (inc a bit of driver side sill bodge fix, it's gonna need welding eventually)
- Front passenger window switch replaced with a working one
- Left hand side of boot paneling wasn't in properly, causing rattling = looked funny & boot shelf didn't fit right
- Deep clean of interior inc removing seats and giving them the Rug Doctor treatment
- Rattly battery container lid solved by attaching with screws (this was super annoying)
- Removed ICE remote control stalk from steering column (kept bashing it with my knee)

Future projects:

- Replace boot light with brighter LED panel(s)
- USB port(s) in rear of center console somehow (I have teenagers)
- Replace all Speakers with nicer ones (£££!)

All while keeping it looking as close to stock as possible


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Led tube things added just in time for Autumn/Winter, they're just held in with double sided sticky pads at the minute, will likely properly screw them in at a later date. Bonus is that I've run both through a switch so you can turn them on/off while the boot is open too (they still auto switch off when the boot is closed)


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