See the pic. My alternator has what I think is a regulator mounted on the top of it. See the red wire going to the post. There's also a plug from this that can go into the alternator. The wire from the wiring harness also has a plug on it, visible in the pic. I can't remember which plug was used when I pulled the alternator a while back. I do recall the red wires from both the harness and small mounted box went to the same post on the alternator.
Tried to attach a pic, hope it worked
'69 El Camino SS
396 CI,Hooker Super Comps
TH400, HEI, Performer Intake, 750 cfm