If they hadn't built that circuit breaker into the switch the diagnosis would be much simpler, just follow the melted wiring!
Or follow the fire. I agree with Jerry...look at the dimmer switch first. Something is grounding out somewhere and that's a good place to begin. Second place to look...a missing grommet on the inner fender where the headlight wires pass through. Years could have worn away the insulation and you could be going to ground right there. Good luck!
