You are missing a ground connection at the light sockets, either the sockets or the bulb base are corroded and not making connection, or your ground wire for the wiring harness is not hooked up for the bulbs. The only other way this can happen is if a bulb is broken and the filament of one falls on the other filament (I have never seen this but it is possible). I always remove the lens on all the turn signal,parking, taillights, so I can view all the filaments. You need to only have one type of light on at a time to troubleshoot this problem, verify the brakes first, with the brake pedal pressed and tail lights off, the heavy filament in the brake lights. Good Luck.
Last edited by rex1965; 08-26-2013 at 09:59 PM.
Reason: more info