Jim, thank you, thank you, thank you!!

That was the missing link! Matches exactly what I have. So I guess the brown lead labeled with "blower" or "blower motor" that comes from the firewall bulkhead connector must be for cars without AC and they just left it in the harness. I'll have to see about tucking it away or maybe removing it and documenting it if possible just to clean things up since this is not a full return-to-original car.

That is a good point about the alternator source for the 12V supply wire. It's already a good size, probably 12 gauge. I was thinking about adding a buss bar near the battery but getting power from the alternator wiring and would have to upsize some wiring to achieve that, but I don't know if I want to go to all that work.

But that diagram answers all my questions, so relieved. Thanks again!