I bought a used 2005 coupe in January and just noticed (when wife drove car) that the spoiler brake light does not work, only the interior brake light. Searching here, I learned about the B421 plug connection under the trunk lid padding. Checking it out, the spoiler light wires were spliced into the wires that had been cut from the plug. Either the dealer used an aftermarket spoiler or the first owner got it somewhere else. I am trying to reach the dealer to find out in case I can get warranty coverage.

I tested voltage with my voltmeter and when I hit the brake pedal no voltage came thru that connection wire to plug B421. However when I used a lawnmower battery for power I was able to power the spoiler light through its wire and the interior third brake light thru the connection wire! When I reconnected them and hooked up the lawnmower battery, both brake lights worked. However it does not work when applying the brake pedal. The brake light fuse in the cabin is okay, I do not know if there is one in the box behind the battery, but will check that tonight. Otherwise, I am at a loss for how to repair this situation. Any ideas?

 

does the third brake light in the window work ?

I'd follow the wires to that one. I added a factory spoiler to my car and the spoiler brake light wires just plugged into the factory wires in the trunk using the clip that was not used comming from the brake/tail light wiring. Works perfectly. Now I just need to unplug the brake light in the window.

 

Yes, I said that in the first line. The dealer advised they apparently did not install the spoiler, although I do not necessarily believe them. I am YOYO for this repair.

 

well damn

my thinking is that if the brake light in the window is working you probably have damaged wires between there and the spoiler based on what you said about the hack job wiring someone did. Check the wires and look for the connector coming from the wiring harness in the trunk. Should be on the pass side of the car. You may have to pull the plastic trunk liner out. Just have to remove the clips with a flathead screwdriver first.

 

The wacky part is when I applied 12 volts to the wire that runs from the spoiler light into the trunk lid harness...the inside brake light turned on!
Indicating there is a valid connection. I am going to pull apart the inside brake light, disconnect the wire and see if the spoiler light then works. It probably won't and I will have to run a new wire from inside the car to the trunk lid.