A 12 volt draw on battery from a one wire alternator

I suppose by now you've noticed both alternators are the same OEM correct Square back alternator. Difference being one of them has a aftermarket regulator tagged onto the back of it. I don't know how your mechanic wired this but if not right it wont work. Originally, the blue wire should power the regulator and the field but only at switch on.
I see a added red hot at all times going to that regulator and one of the field terminals. Don't know where that blue wire disappears to or how the heck that works, BUT.... Since the following is pretty much how older GM charging systems are upgraded, There is possibly a place where your mechanic jumpered/tied the OEM blue and green together. In the GMs that was done with a short jumper with 2 male spades at their OEM regulators harness connector. That jumped only 2 of the 3 or 4 wires ( I forget ) at their regulator. Anyway... Check your blue and green all the way back to the OEM regulator before you plug it in. Make sure that regulator is well grounded at it mounting bolts. Hook it all up and check that your current draw is gone. Start it up and check for regulated output.