Check your grounds.  Clean and make sure they're tight, even if they seem okay.  Esp. check the ones in the taillight buckets.  Those can be a problem.  While you're in there, make sure no wires are touching or rubbed through.  Bad grounds cause all sorts of wierd things to happen to lights and flashers.  I remember my bus's right blinker wouldn't work, and the brakes caused the back-up lights to turn on.  Had to replace the bucket, and it cleared right up.  Good luck!