You're probably dealing with a grounding problem. First, check for proper voltage going to the light, and then be sure the bulb is grounding to the frame- - - -not trying to back up through another wire like a tail light or brake light circuit.
Do you have a wiring diagram for the bike?
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!