Attached is a wiring diagram of a typical charging circuit of your era truck. If you are checking and seeing no voltage at the voltage regulator Bat. terminal, there are a few possibilities: 1. Battery is depleted 2. Ammeter is open 3. Loose wire in that run(s) 4. Your multimeter is defective or you are not using a good ground on the negative lead. If any of the items, 1-3 are true, it may explain why the lights work temporarily (charged battery provides power but gets depleted because it is not being charged).
