WCKaner,

I will go out on a electrical limb on this one. Does your '66 C10 have a printed circuit board dash panel...I think it does! I wonder if turning on the headlights bleeds power through one of the little dash lights and 'cornfuses' the ability of the fuel gauge to see ground and read correctly. Maybe this short is finding ground through the fuel gauge.

If you sit in the cab at night and pull on the headlights, is there a bulb in the dash that is blown out or appears weak! This could be your issue....maybe. I could get blown out of the water with this theory...lets see what happens.

EDIT. Further digging leads me to believe that your dash panel is a conventional wired system, and that I might be way out to lunch.

Stuart