Look very closely at all the wiring harnesses, engine harness, front light harness, rear taillight harness and so on, Look for melted tape Once you find that your on the right tack to fixing it, especially under the dash and the engine compartment The bulkhead connections can corrode and cause a high resistance which will make the alternator over charge, same thing on the back nuts on the amp gauge, they can vibrate loose and cause the same thing
Let us know what you find