1992 GMC Truck electrical wiring

Electrical problem
1992 GMC Truck V8 Four Wheel Drive Automatic

there is a wire that comes from the battery terminal down pass the starter, then goes up into what I think is the cab firewall. The wire either melted or broke, by the starter. We replaced the positive cable from the battery to the starter. We tried turning on the lights to see if there was power, no lights came on. Then we touched the broken wire from the cab firewall to the starter positive bolt and the lights came on. Then we bought a two ended wire tube ( plastic with metal inside for wiring), connected the broken ends crimped it. Then crimped it at the battery terminal. The truck started, we drove it maybe half a mile then it just died. Opening the hood it smelled like plastic burning, any idea what this wire could be called or what to look for next in troubbleshooting whats wrong?