Tom:
I don't know if this is the problem but I have seen it twice lately. Check all of the running, brake, tail, and signal bulbs. If you have any bulbs with two filament check them closely. I have seen two of these type bulb burn out one filament, have it drop across the other and it ties all the stuff together. It can make for some very weird light problems. If you don't have twin filament bulbs then disregard this answer.
Good luck, shorts (and that's what you have) are fun to find...