A 12 volt draw on battery from a one wire alternator

I tried to wade thru the posts, but a bit confusing. The OP never told us the year of his car. It has a later (1971+) voltage regulator and square-back alternator.

I didn't see the thick ALT wire connected to the alternator's big output stud in any of the photos. This is the insulated stud. One alternator has a second ground stud and I saw a wire bolted to that. I hope that isn't the ALT wire since that would short BAT+ to ground, via the bulkhead connector and dash ammeter.

But, he is getting >15 V while driving, so the alternator must be outputting to BAT+ and not shorted. Re using a multimeter and basic electricity, there are many youtubes and wikipedia pages. Re monitoring while driving, get a cigarette-lighter voltmeter to use in addition to the dash ammeter (if that still works).