I see 2 possible reasons. Either there is a break further forward along the harness as it runs along the frame, do the brake and directionals work ok? The other is at the headlight switch itself. Besides the headlight dimmer terminal and the dash lights terminal there are 2 other terminals that get powered when the switch is pulled out. One powers the front parking lights and may or may not stay powered when the switch is pulled all the way out for the headlights. The other terminal is for the tail lights and will definitely stay powered in either the parking light switch position or the head light switch position no matter what age the vehicle. Either work with a wiring digram and a volt / ohm meter or just the meter if you don't have a schematic and you should be able to prove out the condition of the switch. If you find the problem to be in the switch then replace it. Jumping across from one terminal to another might end up cross connecting 2 separately fused lighting circuits and give you something else to repair.
Mike
