Dim Lights

There's a heavy gauge wire going to the starter so a good battery and a starter that spins great is not a clue. The entire remainder of the vehicle operates on a much smaller wire. This wire goes through a service disconnect connector and a fusible link just before the bulkhead connector. Once the wire gets past all these common failure points and to the inside of the cabin, it branches from a welded splice to several more connections such as the fuse box, ignition switch, etc...
so theoretically, if the headlight circuit alone is weak, the problem is likely at the headlight switch. If all the branch circuits are weak the fault is back at the root.
All of that is about the positive side of the system. The negative side could be the fault location. It is much simpler since most everything is chassis grounded. That means the cars metal is a ground wire. The negative or ground wire on the battery goes to the engine and is surely grounding the starter. Because the engine is pretty well isolated from the rest of the cars metal, by rubber mounts, a small ground jumper wire is attached from the back of the engine to the firewall. There are a few more of these small ground wires in various locations that differ with year model. Most of those rely on the main one from engine block to firewall.
As for whether the voltage regulator is doing its job or not, I think its doing as well as can be expected. The longer you sit and idle and loosing stored power in the battery the harder the alternator will work to build it back again. This only shortens the life of the battery.
So here I've typed out a whole bunch of crap that should help. If you decide to just take it to your local garage and let them fix it, I would rather not know.
Cheers