Something I used to do is when a customer brought in an alternator for this reason is test it, then if it came back good, I'd tell them to re-install the alt, make sure all connections were clean, and bring the car down to test again. By testing it in the car (or truck), you will be testing the wiring and grounds too. A good way to narrow it down a little. Then I can look at things and make suggestions for fixing. Helped a bunch of people by doing it that way and got the issues resolved.