Alternator

1986 Camaro Z28. I have replaced my alternator 3 times. The car in on it's third Battery. I think the battery are being drained because I either don't have the alternator hooked up correctly or there's a problem with the alternator holding a charge. From what I can tell, the amps from the battery are pulling about 25% while running and the alternator will not pull the lights while running at night, it drains it immediately. Someone told me to disconnect the positive post of the battery while running the engine and if it goes dead, then the Alternator is bad. It does go dead but everytime I return the alternator they tell me it is passing the test. I'm at a loss for what to check next. Any thoughts?