1983 Cadillac Fleetwood 1983 Fleetwood alternator

Hi I recently changed the alternator and battery in the first I went and bought the battery to get the car home because it completely died and wouldnt hold a charge when I try to jump it. Then I got the alternator and still couldnt get the volts to 13.00 so I got another new battery and got it to 14.0 for a little bit when it was idling but slowly went down went to 13.5 then back to the 12.5 then drops to 11.60 after a while then dies. I went to the place I got the alternator and they tested it and said it was good so I put it back in took the car there and they said they didnt know the test that they had said it had low volts low reg something like that. Is there a volt regulator somewhere in my car or a something that could be causing this if so where would it be located? Im taking the alternator back later to just get a new one anyways but dont want this to keep happening. Thank you