OK. In my experience, the starter frame is grounded to the engine block by simply mounting the starter to the engine/transmission bell housing. Can't screw that up. After that, there's two or three screws on the starter solenoid, and here it IS possible to get something wrong. The first screw has the big cable from the battery, and nine cases out of ten there will be a forest of other wires that connect here as well. It's the main connection between the battery and the rest of the truck. One screw will have the wire that goes to the starter switch, it will be the only wire on that screw. All it does is trigger the solenoid so the starter will engage and start the engine. Then, if it's a remote solenoid you will have a third screw that has a heavy wire that feeds the starter. The only time this wire carries current is when the vehicle is starting. If the solenoid is built onto the starter, this circuit doesn't have a separate wire-- it's internal, part of the starter.solenoid package.
Somehow, somebody crossed a wire so the starter is always engaged/trying to engage. Yeah, if that happens you can heat up the starter and get things toasty warm. It was never designed to be always engaged like that. Look for the wire from your key switch to have been placed on the main battery cable, and/or one of the other wires feeding other circuits to have been placed on the relay switch terminal. Something is keeping that relay engaged ALL THE TIME, and that's why the truck caught fire and burned.
