While I'm thinking of it ...
When your taillights act weird, it's usually due to a faulty ground in one of the lights.
Most incandescent taillights use dual element bulbs ... one element is for running lights and the other is for brighter stop and turn signal. The base of the bulb is the common ground for both elements and the two silver-colored buttons on the bottom each provides +12v to it's respective element.
If one of the STT (stop/turn/tail) fixtures has a faulty ground, the current goes through running light circuit to the other fixture to reach a ground.
Fix the ground on the faulty fixture (or both fixtures to be sure) and the problem should go away.
