Measure the current draw on the battery and then pull the fuses one by one, when the current drain stops, you have found the circuit that is pulling the juice.
With my bimmer, I found that the trunk light was not turning off when the trunk was closed, thus with the car sitting for extended periods of time and the 5 watt bulb burning for days, it would eventutally run down the battery.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record