Reddit reviews Extech 380942 True RMS 30 Amp AC/DC Mini Clamp Meter

Two common causes for this in the E46:

1. Defective aux fan
   
   
2. Defective HVAC controller module
   
   
3. Defective final stage unit (HVAC blower motor speed controller)
   
   FYI, the proper test equipment for troubleshooting battery draw is a clamp-type meter that has accurate measurement in the mA range. I used the following on my car:
   
   http://www.amazon.com/gp/product/B00023RU0S/
   
   The key to testing this is to let the car go to sleep, at which point it should be drawing no more than 35-40mA, and then pulling fuses. This obviously requires the passenger door and trunk to be left wide open so you can monitor the draw on the meter and enter the vehicle to pull the fuses without causing the car to wake up. Some people say that merely pulling fuses will cause the vehicle to wake up. I never got that far so I can't relate whether that's true or false. But unless you have access to BMW SSS, which can allegedly report draw of individual circuits, even historically, that is the process required.
   
   You should also consider the possibility that the battery is just toast. If it's older than 2-3 years and you don't drive every day for at least 15 minutes for each turn of the starter the battery may have a bad cell and so any draw takes it under quickly. Seen that many times. You should have 12.6V +/- 0.2V on the battery with the car off, and 13.8 with the engine running.