Check for voltage drop across the contacts of the headlight switch and the dimmer switch. Dirty switch contacts can rob a lot of voltage before it gets to the lights. Ditto for grounds, particularly back at the tail lights. Running relays to the headlights takes a lot of load off the switches. It's very noticeable on 6 volts, but there will also be a difference with 12V headlights.
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!