I believe on the later models the two white wires, about 10 gauge are the alternator wires. At least one goes to a junction block on the fender, then goes across the core support and over to the fuse box to either a 60 or 80 amp buss fuse. Then there is also a couple smaller gauge wires to the alternator, I don't recall the colors. Just follow the harness from the alternator out and make sure everything is ok. Some trucks the harness winds up laying on the steering shaft and could chaf the insulation. 15 amps won't run much, so if you have the engine and lights on, it may not have enough juice left to charge the battery.
There should be a ground wire on the back of the head, one from the block to the fender, and one from the fender to the battery.
