Ford: 1990 mustang 5.0 electrical (battery not charging)

Question
Had Advance Auto check it out, they said it was a bad alternator. Under a load it only had about 12amp output. I replaced it only to have the same thing happen, which was the battery draining after sitting a couple of days. They say the battery is good. I had a similar problem 2 years ago when battery would not hold a charge. I replaced the ground cable and it solved that problem. Any ideas besides taking it to Ford and paying a fortune to have them run diagnostics?
Thanks!
Jim

Answer
Hi the best place to start is disconnect the negative terminal, connect a test light between the cable and battery and start pulling fuses till the light goes out. If you have a volt meter. That reads AMPS. Do the same using the meter just gives you a better idea of the kind of draw and size you have. Try this and get back to me