Before you dash off to the parts store, clean up the area where the regulator bolts to the firewall or better yet, run a ground wire to the regulator just to rule out a bad regulator ground as the problem.

Another obscure cause might be a bad ammeter connection. My father had a 75 Gran Fury when I was growing up that would fail to charge after replacing alternators and regulators. He blamed the "new fangled" electronic regulators as the culprit and switched it back to the mechanical style regulator and it never gave another minutes trouble.

Fast forward a few years and one day when he was into the dash to change something, found that the ammeter connection was IIRC just a push on friction connection and it was as green as grass. Cleaned it up and the electronic regulator worked fine. The only thing we could figure was that the regulator saw the resistance from the green corroded connection as a full charged battery and never turned on the alternator.

Kevin