When I leave the vehicle for more than a 3 weeks the vehicle battery goes dead

More than 3 weeks is a pretty long time. You could wire in a main battery power cut off that was accessible with out lifting up your rear seat. Most good shops could handle this. http://repairpal.com/bmw-repair-in-clayville-new-york

the battery is located in the trunk on the passenger side near the spare tire. you can access it from there. there is a battery cut off switch for long term storage located in the passenger side trunk compartment access door. once you do this you will need to open the trunk with the key that slides out of your FOB.