Fixing taillight issues on the Audi TT

The Audi has been experiencing issues with the left taillight. Mainly these appear as brake light signal leaks and a constant symphony of "Light failure" CAN beeps. The problem appears when you press the brake pedal. The signal leaks into the left signal light, fog light and parking light, for example.

I first tried cleaning the connector, but the issue is more complex. The ground pin in the connector is really oxidised and cleaning will help only so much. The wire in my car's scheme is brown but if you have the same issue this is very easy to spot as the oxidised pin will look rather nasty.

The fix here is a new ground wire. The taillight comes off easily, with two finger screws. The wiring frame is attached to the light with two screws. I soldered a bit of wire to the frame's ground circuit. I then routed the wire from behind the frame and into the car's frame. Using a cable mount here.

The wire is soldered to the taillight assembly but attaches only with a screw to the car frame and is in a place that is accessible through the small opening in the trunk lining.

Supplies used: spare piece of wire, multimeter, soldering iron and soldering tin, one spare bolt. Time: about 30 minutes, most of which in searching for a bolt of the right size.