The voltage regular is mounted on the firewall near the master cylinder. mechanical or solid state depending on what you are working on.
The first thing to do is have the altenater and battery tested.
All of the cars electrical goes through a fusible link at the firewall then the bulkhead connecter then the amp gauge then back to the wiring harness where about 5 wires are all bonded together near the inside of the firewall .
Any of that could be the problem. There are web pages that suggest an added wire under the hood to bypass all of that. Its a cure for the symptom and easier than hunting the actual weak link in the circuit.
If that doesn't cure it, the problem could be just a weak connection at the headlight switch or a weak ground somewhere.
It looks like the old style alternator