I have had these sort of problems a few times over the years, and the fix usually begins with unwrapping the black tape to more easily follow each wire from the firewall bulkhead out to its end.
Most recently, I had a similar situation with a 74 Dodge truck. Most of the wire colors are the same for the cars and the trucks. Models with A/C and cruise control will have more wires than a slant six, 3 on the tree stripped down model.
With my truck, the terminals in the bulkhead connector had pulled out slightly and allowed some corrosion to form. This is a problem in two ways: The larger gap between the terminals makes for a poor electrical connection, AND the corrosion just makes it worse. When the gap gets too wide, the wiring either melts or just fails to transmit any juice period.
My friend and I unwrapped the tape and traced each wire, looking for splits in the insulation, as well as any melted areas. We found a few, but the real problem was in the bulkhead. Some may suggest to bypass the factory amp guage, and this can be good advice. They can ground out or stick open, leading to melted fusible links or an outright wiring harness meltdown.
Good , clean connections, proper wrapping with the black tape, a known good battery-alternator-voltage regulator and you should be ready to go.
