Regulator gone bad?

How much of the under-dash harness is original? Even though you have bypassed the ammeter, some of that wiring under the dash is still acting as the battery buss

Your most probable areas are:

recheck the bulkhead connector (interior) on the battery feed

If that's OK, then your most probable are the ammeter/ ammeter connectors (which may be jumpered?)

ALSO there is a great big splice in the ammeter/ battery feed circuit IN THE HARNESS. This is a factory "welded" splice, taped up in there that junctions the ammeter, and the hot feed to the fusebox and ignition switch.

I HAVE FOUND three or four of those which failed over the years.

You are narrowing it down, stay with it.