This definitely sounds like a ground problem but it doesn't have to be where a wire is bolted to the body. Suspect a corroded or stretched contact in one of the light sockets. One way to verify that is by removing other bulbs. What happens is the current going through a bulb with two filaments, (brake / signal and tail light), goes through the brake light filament, can't get to ground, so it goes to the tail light filament, and once on that circuit it goes over to the tail light on the other side, through that filament, then to ground. By removing the left bulb for example, the right one will go off completely.
Once you have all the bulbs removed, put one at a time back in until you identify the one that doesn't work by itself.
A more difficult way to diagnose this is to measure the voltage on the grounds of the bulbs themselves. That can be hard to access with the newer plastic base bulbs. It was much easier with the older brass base bulbs.
Saturday, May 7th, 2011 AT 3:59 AM