There is a main junction mounted on the firewall, just to the passenger side of the brake booster. If you disconnect the wiring at that point you can separate the engine/starter wiring from the wiring running to the rest of the truck, and then start to troubleshoot.
- If you find a short to ground in the wiring running from that junction back to the starter and battery, the short may be inside the metal tube that is used to protect the wiring from the exhaust heat.
- If you find a short to ground from that junction going forward to the rest of the truck, you have a lot of areas to start troubleshooting. I think I'd start out by pulling all the fuses and see if that isolates the problem. If pulling fuses won't fix it, then you have to look at one of the main feeds that have a fusible link or a separate fuse (high speed blower is the only one I know of with its own fuse).