Check both tail/brake light sockets for grounding. One side will show a high resistance, or no ground at all. Rusty mounting hardware or a bad connection between the bulb socket and the housing is the most common cause of what you're describing.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!