So there are three sets of wires that go to the starter. One large black one that runs from the battery positive terminal down along the oil pan to the large post of the starter. Also connected to the large post on the starter will be two red wires that fuse together into one connector. These two red wires are fusible linked and are the main power feeds for the truck electrical. The third connection is a small wire that goes to the solenoid on the starter. It is what triggers the starter to spin.
The two red wires and the solenoid wire are loomed together and run over the back of the engine or bellhousing towards the driver side. Behind or near the distributor the loom will be T'd and the alternator wiring and choke wiring will join the loom. The loom then runs under the brake booster and into the firewall connector. At some point between the distributor and the brake booster section of the loom, one of those red wires runs to the fusable link mounted on the firewall just left of the brake booster.
Your truck may be different, but this was how mine was run. Read through the wiring diagrams (scan are posted in multiples places online) and look for those wires for your year to see where they should connect. A multimeter is helpful for continuity tests to confirm you are looking at the correct wire on both ends.
Good luck.