First of all test the batteries during cranking. Keep pos and neg cables on and with your mulimeter lead on each post have someone crank engine. Just sitting should be around 12,6 volts, if it drops below 11 volts while cranking your batteries are weak and need replacing. For testing the GP system I have a tech write up in the maintenance section, but when the key is turned on you should here a loud click and you should have 12 volts at both large studs, the rear may be 11 volts but that's okay!! If you don't you have a bad GPR and it needs replacing. For a stock replacement use the GPR #109 from Napa, you will have to rotate it 180 degrees to mount but the wires fit fine. It is less than half the GPR 110 that Napa's book says is for our trucks. The best replacement is the heavy duty Stancor replacement!! It's what I suggest everyone to use!!
If all this checks out you need to ohm test each gp.
NCH