Head light relays were added when a truck was loaded up with extra lights that came on with the headlights. It was easy to overload the head light switch, which would then shut off all the lights after some indefinite time period, somewhat dependent on high/low beam useage. They would stay off until an internal breaker reset, also an indefinite time period- then you could go on your way. Until the cycle repeated. Head lights going out is unsettling enough when you are on a freeway with other traffic to light your way, but it really focuses your attention if it happens on a mountain road when the last thing you saw was a curve ahead.
I don't know if your truck has a circuit breaker in the switch, but newer switches have them. I seem to remember that yours has a fuse on the back of the switch.
Harvester
