Could your problem be a bad bulb? I had a tail light/brake light problem I couldn't find, an elderly observer said "you got a bad bulb", A new bulb fixed it.
Good luck,
Roadster, 31
I found it. No ground on the passenger side. I ran a wire from the bumper to the base of the bulb socket and bingo. Only one filament came on, and same with the other side.
