Does the fuse only get hot when lights are on "dim" ? If this is the case it's probably the resistor coil on the switch. Seems the coils sometimes(or maybe always) can't take the 12 volts. You could replace with a 12V switch. The 350,400,450 diesels were 12V systems, although I don't know if you can buy new ones. Another idea is, moving wire from "low beam" screw on back of switch to the "hi beam" screw. By doing this you'll have "bright lights only" in both positions of switch. I've done it this way before, low beams are not that useful anyway and no one will know the difference, except you. Incendentally, the part # for a 12V switch is 363131R91, if you decide to go this route. Good Luck!