Upper Third Brake Light not working

[ronkimes]

Failed state inspection due to upper third brake light failure on my 04 V70. The upper third brake light is located in the spoiler.

Some background:
Three years ago my alarm kept randomly turning on and the door locks were acting erratic. I was able to troubleshoot it back to the wires concealed in the plastic housing in the hinge for the trunk door. A few wires were frayed or absolutely broken. I repaired them and all has been fine since then.

To the upper third brake light:
I am unsure if the upper third brake light has worked since the issue with the wiring in the hinge. I never knew the light to not be working, but it is absolutely possible that is hasn't worked the past three years. One of the wires broken during the incident three years ago was a yellow wire that supplies power to the upper third brake light. I traced this wire down to a connector above the driver rear speaker. It then traces to a yellow/red wire that is connected to the rear electronic module.

Upon recommendation of a mechanic, I purchased a new upper third brake light led strip and attempted to simply install it. I removed all surround plastic panels in the trunk area and I unplugged the green two wire connector in the upper area of the trunk door (where the upper third brake light directly plugs in). The connector has one yellow wire and one black wire which connects through a green connector to one red wire and one black wire for the upper third brake light. I prepared the new upper third brake light and temporarily installed it only to find it would not work.

I ran temporary positive and negative wires from the battery directly to the upper third brake light' red and black wires and the light turned on (also performed the same for the new upper third brake light and it also worked). The light appears fully lit. I disconnected my temporary wiring to the upper third brake light.

I then checked for voltage at the green connector I mentioned previously, while it was unplugged to the upper third brake light I received a range of 7.5 to 8 volts. I then plugged in the upper third brake light and tested for voltage and the green connector: I received .24 volts.

I then had a helper press the brake pedal in and I received .24 volts again at the green connector. I unplug the green connector and my helper pressed in the brakes I receive 7.5 to 8 volts (obviously no voltage to the wires on the side of the connector which lead directly to the upper third brake light).

I checked for codes using a generic code reader, but there are none. I checked every single fuse in the engine compartment, the compartment to the side of the dash, and the rear cargo compartment. Every fuse is rated correctly and none were blow. I removed the positive terminal from the battery as a last line of wishful thinking... The upper third brake light still dot work when the brake pedal is pressed.

This is where my knowledge of troubleshooting this issue ends. Any further help is much appreciated and I am absolutely open to anything. Thank you. Ron

[ronkimes]

Also performed today:

I tested it just now with the connector unplugged:
8.5 volts connector unplugged
8.5 volts unplugged with brake pedal pressed in

With the connector plugged in:
.24 volts plugged in
.24 volts plugged in and with brake pedal pressed in

I am able to trace the yellow and black wires down through the trunk hinge into a gray connector with multiple wires and I have continuity for both wires at that point The yellow wire then becomes a yellow/red wire and I trace it to the rear electronic module and I have continuity of that wire. I can trace the black wire down into a white plastic channel under cargo pop-ups (the trunk floor, I guess). From there the wire extends towards the front of the car but I gave up disassembling the car because I thought if the wire extends to the front of the car, then I would just test it there. But I have to locate where the black wire begins. At the central electronic module?

[mikealder]

Thanks for the PM, My first guess is the electronics in the REM feeding the centre brake light has blown when the wiring broke/ shorted out inside the tailgate hinge. All is not lost though as you now know the wiring is good, you can use an automotive relay to switch power to the centre brakelight, trigger the relay by wiring the coil of the relay to one of the wires feeding the outboard brake lights.
This modification/ repair is similar to one I wired in to a phase 2 V70 a couple of years ago, in this instance the CEM had blown and burnt out quite a bit of wiring in the rear of the car, I know your CEM is working as when that fails you get no brake lights at all, the signal from the CEM to the REM is intact otherwise you wouldn't have outboard brake lights which points at the electronics in the REM, changing the REM is costly so I would use a relay to drive the centre brake light in the spoiler - Mike

[ronkimes]

Mikealder,

Thanks so much for the reply. My fear is exactly what you stated is your guess: that the broken yellow wire in the trunk hinge has created a fault in the REM. I had looked into replacing the REM and it was a few hundred dollars, which I thought wasn't worth it. I had performed work to the vehicle right before I read your post and this was my solution:

I reasoned that I could use the trailer wiring harness and tap into it to control the upper third brake light. I was able to locate the 12 volt wire in the trailer wiring harness and the ground wire and verify power. Had my helper press on and off the brake pedal and was able to verify that I was receiving 12 volts when brake pedal was pressed in.
I simply ran new wires from the upper brake light down through the rear driver's side trunk hinge and all the way to the wiring harness. I also wrapped the wires in that hinge with a bicycle tire tube as sheathing. Then used some quick connectors/disconnect and tapped into the trailer wiring harness for the upper third brake light. Thought it might be useful in the future if I ever needed to quickly disconnect the upper third brake light (if, say, I was using a trailer and was concerned the trailer lighting would overload the circuit with the upper third brake light also in operation).
I would have liked to have solved the issue of the upper brake light not working, but I was spending a lot of time and I began to fear that there may have been damage done to the Rear Electronic Module a few years ago when the wires in the trunk hinge were broken. I hope my solution is long lasting.

Thanks!