Battery dies after auto being idle for 2-3 days.

[spinedoc]

1996 SL500. I have had a very weird problem the last couple of months. If I leave my auto sitting without starting it for 2-3 days it will not start. If I leave it for a week or 2 then the battery will be completely dead.

I would assume the alternator is working because once I jump it and run it on the highway the car starts fine as long as I keep it started once a day. I can only assume something is drawing power, but am not sure what. The only aftermarket thing I have is my head unit and that has been installed for almost 2 years and I have never had any battery issues.

I even bought a new battery from the dealership and installed it, same thing. Any help would be appreciated.