Exactally what he said^^, pin # 15 on the red connector for the ECU will have a brown wire that usually is the only wire that inputs the truck alt. sometimes there are two small alt. wires from the 5.3l harness, a brown and a pink, if there's a small pink wire that shares the same clip as the brown wire, it needs 12 volts to make the alt work. you can also use this for the "batt" or "alt" light in your dash, this would be the small original wire that went to your old alt. the big power post on the truck alt. needs to run to the batt., its your power wire to charge the batt. that usually goes to the big post on the starter, you should also have a bigger pink wire (factory main hot wire) that comes out of your firewall that is used to power up the cars dash and all the accessory's, this bigger pink or red wire also went to your original alt. or starter, it needs to go to the output of the 5.3l truck alt. or the big post on the 5.3l starter.