First thing I would do is a thorough inspection of the wiring. Follow the wires from the headlights back to the switch. Look for places where the wires could be grounded by chafing against metal. Look for areas where wires could have had the insulation burnt off by hot parts such as the exhaust. Look for wires spliced together and wrapped up in tape (possibly highlight pigtails); a good place for short circuits.
Reread Hotrod Lincoln's post about disconnecting the wires at the headlights and working back to the switch.
