Take a look at the dimmer switch. It lives in a very hostile environment with road splash, dirt and salt, and the contacts inside it corrode and get intermittent high resistance situations which translate to dim and/or flickering lights. Setting up the headlights to run on relays and just using the dimmer switch to turn the relays on and off is the best solution to the problem.
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!