Battery draining overnight

Headlight switch is not fused but has an internal circuit breaker. It is wired through the splice in the underdash harness coming from the ammeter - at least that’s the way it works on my 65
This is remained correct for most A-bodies and options until '75/'76. '76 moved some of the power distribution splices to the engine compartment side instead of using one main splice.

But what does this mean?
It means the output stud is partially grounded.
Need to look at the alternator to see how it managed to do that.
So what you’re saying is I need an alternator
Yes.
Or fix that one.
New does in no way shape, form or fashion equate to good.
Exactly.
Just this is year I bought a brand new AC Delco alternator supposedly to GM specs right down to the part number. (This was for my '85 AMC - just so you don't think this was Chevy application, LOL). Output stud came loose after a weekend in NY state - and fortunately I had just enough time to make it back to Phila in daylight. Opened it up and the insulator was misformed so the stud was cockeyed and therefore it was inevitable it would loosen up. Damn good thing it didn't short! Yes they took the return and exchanged it. Still totally BS. Never would have been accepted in real production.