Blowing voltage regulators

It very possibly could have grounded on the case. I also have no clue what this third wire is supposed to connect to.

View attachment 1715458222
What year, etc ?
Could be temperature sender.
Could be power to horn relay.

I didn't quite follow all that, but I was thinking of just redoing the whole harness with this Speedway Universal 22 Circuit Wiring Harness
IMO that's a whole lot of trouble!
Like someone else posted, that's for someone who completely reworking a car. You know - the guys who want a new car in an old car shell.

Where in Del are you? (you can send me a PM if you don;'t want to post that). if you're not far I could run down if needed