Dimmer switch contacts can get corroded and still show voltage, but lack the ability to carry enough current to burn the headlights bright enough. Try spraying contact cleaner into the switch contacts and cycling it several times. I'd recommend using a couple of Bosch-type relays to power the lights, and use the dimmer switch to trigger the relays. You can run a large diameter wire to the radiator core support area and place the relays close to the headlights so they get full battery voltage without having to run the circuit all the way to the cab and then to the lights.
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!