That adds a lot. My guess is the license lamps don't work either. The brake / signal filament and tail light filament are in the same bulb, and they share a ground wire. When the ground wire is broken current can't get through either filament when there isn't a complete path back to ground. It looks for an alternate path, and that is though the tail light filament, then through the front running lights. If you look closely, you are likely to see the front running lights glowing dimly when the brake pedal is applied. The brake lights won't be full brightness either.
With the headlights / running lights turned on, voltage appears on both of the rear filaments. With the same voltage, (electrical pressure), on both sides, no current flows so the brake lights no longer work.
Start by inspecting the wires in the rear. To verify a ground wire is broken, use a voltmeter or test light to test the voltages on one of the rear sockets. I can't remember if you have the standard round brass base bulbs with two terminals plus the socket that is grounded or the newer four-terminal rectangular socket. If you have the bulbs with the round base, turn on either the brake lights, tail lights, or both, then measure the voltage on the side of that bulb. There should not be any voltage there. If there is, the ground wire is broken. If you're using a test light, you'll see it glow dimly, and the brake light bulb will be dim too.
If you have the plastic base bulbs, remove the bulb from the socket, then see which terminal has voltage. The one across from it should not, although it may get voltage from the bulb on the other side. If you remove the right bulb, then check again on the left socket, only one terminal will have voltage with the brakes applied, and another one will have voltage with the tail lights on. When you reinstall the bulb, the other two terminals should not have voltage, but they will if the ground wire is broken.
Sunday, January 26th, 2014 AT 6:11 PM