I would think that the ignition switch was removed from the circuit. When you turned the key off and the truck kept running, the switch was not doing what it was supposed to do. Then you got grounded out somewhere along that wire between the switch and the ammeter and that lead to the melting of the insulation on that wire as you describe. Perhaps not in that order but if you could not use the key to turn the truck off, it was out of order somehow. Ralph
