Swap the alternator. I had a Mazda B2200 that would kill the battery when it sat for more than a day or two. While it ran it would charge, but one it was shut off it would show a draw. Swapped the alternator, and it fixed it. You may want to check to see if a trunk light or something is staying on when it should be off. That's a big one for VW's and older Audis. Notch the switch, and then it's all good.