I am not familiar with the headlight switch on a Task Force truck, but the Art Deco and Advance Design switches have a bi-metallic circuit breaker in them. On my '42 I have experienced sudden loss of headlights on a dark road. You also get loss of the dash lights at the same time, which would not happen if the problem is with the dimmer switch.
Beltfed, Cletis is correct and what happens is dirty contacts lead to resistance, which leads to heat, (as well as current loss as you point out) which opens up the bi-metallic contact points.
Kent
