Alternator or wiring?

I would almost bet you've found the problem

On a side note, a friend with a 68 RR had a similar problem way back in the early 70's. This was my Navy days at Miramar. I ran it down eventually to the welded splice. There we were, he thinking / wondering if I was insane, tearing his dash apart, untaping the harness.......until we found it!!!!!!!!!!!!

That was the very first failed splice I found, and have been a party to some 8 since, in one form or another. This of course, was way WAY before Al Gore invented the internet, LOL

At the time, all I had to go on was my 69 (I had owned) and my 70 RR service manuals.