Hi Tona, if your headlight wiring is routed through the bulkhead/firewall behind (i.e.. cab side of the air filter in the engine bay), the point at which it comes past some sheet metal on that cab side of the air filter the insulation can fray from engine vibration.
You may have to remove the air filter to see what I'm writing about.
If the insulation has worn through at this point retape it and enclose it in some split corrugated plastic tube for protection.
I know this because I had indicator wiring insulation fail at this point.