I don't have a schematic of this truck in electronic format, but it is rather a simple bit of circuitry.
What is the problem you are having?
Without any further information, I am going to guess you have a battery that dies intermittently, and yet you are sure the battery, cables and alt are all good and test good on the bench.
Your problem is in the plug connector at the alt, and is a broken wire internal to the insulation, replace the plug and about 3 inches of the wires.