So, you only played with the 3 wires at the starter solenoid relay switch on the firewall by the battery? If so, disconnect the Neg battery cable and go to the starter and inspect the cables going to the starter solenoid on the starter. Odds are the cables are corroded. Because of the increased resistance caused by corrosion the cable heated up and may have melted some of the insulation. If you have a booster cable clip it to the large terminal on the Starter solenoid furthest from the starter. Be careful to not touch any of the surrounding metal with the clamp. Clamp the other end to the positive battery terminal. Oh, re attach the neg cable to the battery. Now try starting the engine. If it doesn't start carefully short the clamp at the Starter to the small terminal on the starter solenoid at the starter. The starter should engage. Be sure the truck is turned off so the engine doesn't start. I strongly suspect that the cables to the starter are corroded. It is a common problem on these trucks. The factory nut is of a different metal and it promotes corrosion. There is a TSB out there somewhere with the procedure to change the nut to a copper one.

Regards

Jean Marc Chartier