I bought this car about a month ago with no stereo and the harness were gone, ran just fine until someone tried to install an after market stereo. Now the car will start just fine if the battery is charged, but wont get a charge from the alternator. So I then got another alternator and put it in. Same thing happened. Had the old one tested, napa said that it was just fine. Got a new battery, replaced the fusable line. Replaved the ecm and still same thing. Maybe a short somewhere or? Idk what the problem is but I'm at a loss here. THE ONLY TIME ITS DRAINING IS WHEN THE CAR IS RUNNING. I can leave it on the "on" position and and leave it there for over 10 mins (because I have to reset the theft system) and still have plenty of battery to start the car.

 

Disconnect the aftermarket stereo and see if that helps.Since the problem started when the stereo was installed, it seems the logical place to start.