On something that old, rust is going to be your worst problem. To aid in trouble shooting, get you a 20 foot long, 14 gauge wire and hook it right to the ground terminal on the battery. Use this wire for a testing and not the frame or body. With out a ground nothing will work, and if you don't know if its a lack of ground or power, it makes for some long days. Get you a cheap test light or make your own. Have it hooked to the test wire at all times. If you have positive power at the light fixture and a good known bulb, it should light up when touched with the ground wire.
If you think about all the connections tail lights have to go through, you will start to understand the rust problem. For example, battery to frame, frame to bed hold down bolts, bolts to bed itself, bed to tail light bracket , light bracket to light housing, housing to socket, socket to bulb base. And thats just the negative side!
Listen to Bill in the above post, don't mess with the regulator with out a volt meter and a good understanding of how they work. In short, the regulator supplies the proper voltage to the battery, to much or to little can be deadly to the battery.
You may want to look in to a new wire harness, they are not that costly and can save you a lot of problems. They are easy to swap out if you get one for your make and model. Joe
