car repair help

Have a buddy apply the brakes, turn on the lights, use the blinkers, in every combination. With the harness removed at each light, test each lead, one by one, with a cheapo volt tester. You're checking for voltage and a good ground at every bulb contact. You need to do the methodical testing to find it. It could be a bad socket, loose ground, anything. Worst case is that you have no voltage at the bulb, and that means the brake/turn signal relay is bad, and it's not hard to replace.