Do the lights work with the engine stopped? It sounds like you've got a short somewhere in the lighting system that kills enough voltage when the light switch is turned on to starve the coil for current. I'd suspect the dimmer switch.
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!