I have a 1990 Volvo 745 and the passenger side brake light doesn't work. The brakes are the same as the tail light and that works but as soon as I press the brake pedal, the light turns off instead of brightening like it should. Any ideas? Thanks!

 

possibly a sketchy ground at the right side tail light assembly ?

 

the other lights work and they have a common ground. the light bulb itself works but just not when you brake.

 

The problem is in the socket to tail light connection. The plastic melts, especially if aftermarket fixture, and distorts the contact points... Look closely at how the surfaces mate, you may have to take it all out, not much space in there to see what's going on...