The 30 amp fuse blows, I looked for a short in the wiring no luck. I wiggled the marker lite wiring trying to get it to short and it did not short or blow the fuse. I can run the truck parked for one hour go for a drive on a ruff road and the marker lites stay on. Today the truck arrived at job site marker lites worked Ok, parked truck running with lites on, looked at it 1/4 hour later and marker lites off, the head lites on, 30 amp fuse blown, I put in three 30 amp fuses one after another all blue. Is there a resistor or other electric device that control current to marker lites that may be bad? Could the light switch in cab be bad? Any suggestions would be appreciated.
Nope, will need to pull the lamps one at a time and inspect that wiring, then look into the kick panel Left Side where the dash meets the Door/W/S pillar and check where it transitions into the pillar.
You mention “1/4 hour later and markerlites off, the head lites on” Does that mean tail lights and dashlights out, too? If your marker lights are controlled by headlight switch, you may have more wiring to check... along chassis, trailer plug-in, etc. They have thermal breakers that will plug in to blade and glass-type fuses. That might save you some fuses while you’re hunting down the short.