i have a 1977 f100 the front turn signals work perfectly even with headlights on. drivers side tail light works perfectly with the turn signal, brake, and headlight. the passenger side tail light works for the brake and turn signal and headlight but not all at the same time. if i have the turn signal on and hit the brake its like they cancel each other out and the filament does not light up. i took off the assembly and they all light up the same filament (dual filament bulb) is it the turn signal switch?

 

Possibly... my gremlins came from the switch in the column.

Definitive diagnosis was if I just ever so slightly lifted the switch lever the brake light came on was working.

 

It could be the socket. If you have a multi-meter, you can check for 12V+ from each wire which goes to a filament. And also check to make sure the socket is actually grounded. There should be a ground wire to the body behind (actually in front of...) the tail-light assembly. Your symptoms sound like a bad ground, or perhaps the bulb is in the socket 180º backwards.