No Power to anything

71 Duster with a 340. The problem-no power to anything, no dome light, no headlights, no wipers, no ignition power. The car had not run for 12 years. I just rebuild the engine and put it back in. Went to start it up and had no power to anything. One day I go out and there was power to everything. It was a miracle and had not done anything to it. It was like that for a couple of days and now it is back to having no power to anything again. I know the battery is good and the connection to the battery is good. I have a good ground from the engine to the firewall. I do have power to the starter relay. I studied the wiring diagram and read several threads on here. I wanted to run my conclusion by you experts. With my test light I followed the red wire that go's to the ammeter. I have power at that red wire side of the ammeter but no power at the black wire side of the ammeter. Could this be my problem? Can I bypass the ammeter by connecting the red wire to the black wire terminal?
Could be bad positive battery cable. When I bought my son's Dart, it started fine. Stopped for gas and wouldn't start, nothing. It was bad positive cable.
Had it towed home and the next day my son was messing with the cable and it started.