BMW Repair: 2001 BMW 325i Battery problem, aluminum heat sink, car radar detector

Question
Hi..I have a 2001 BMW 325i battery issue.  In jan 2006 I changed the battery...worked well until Feb 2007, when all of sudden the battery died.  I had the battery checked and had the electrical checked in the car.  I decided to replace the battery and two/three months later, the battery has died again. I have had the car checked again, but they were unable to find anything.  They checked the fan to see if anything was wrong as well as other electrical issues. They worked on the car a couple of days - starting and stopping the car numerous times - but were unable to find anything.  I took it back to work and when I left the office 4 hours later, the car had died again.  I'm baffled as to what it could be.  I did replace the batteries, both times with a CarQuest battery as oppossed to a BMW battery (slightly different size)- should that make a difference?  Are there other things that I could check on the car? I don't want to change the control module, without knowing more.
 Hoping you can help.

thanks

Answer
Vipul, make sure you don't have ANY aftermarket devices installed in the car. Radar detector, aftermarket audio devices, sattelite radio, cell phone, ANYTHING. Keep it bonestock. Have the dealer do a closed circuit current draw on the vehicle and make sure the car is not drawing too many amps and is entering sleep mode properly. If not then have them diagnose the draw source. If it is sleeping okay. Have them remove and inspect the A/C final stage resistor to see if it is the older cast iron style unit or the newer aluminum heat sink style. If found to be the older iron unit replace it, charge the battery and take the car to see if issue is resolved. Let me know what the current draw is and if the A/C final stage was the cause.
-Evan