Sometimes I had the climate system blower in my BMW X5 slow down so it could barely be heard, even when increasing the fan manually to a higher setting. After some minutes it usually started behaving normally again. Yesterday though it simply stayed on minimum no matter which buttons were activated.
After a bit of reading I determined that the final stage resistor (aka final stage unit or blower motor resistor) was probably the culprit. This morning I went to the BMW dealer and bought the original unit for €90 and replaced it myself in about 15 minutes without any issues. The most time consuming part was actually to get to the dealer and back in rush hour traffic.
The BMW part number is 64116923204 (PN 64 11 6 923 204) and is a newer, slightly different formed, and hopefully improved version of the existing unit. You can also buy the same unit cheaper at Ebay but from a different manufacturer. Simply search for the part number in the auction description.
There are some excellent guides full of pictures to follow which you should have a look at:
- http://www.bimmerfest.com/forums/showpost.php?p=1683478&postcount=1
- http://www.xoutpost.com/articles/x5/electronics-x5/9471-final-stage-resistor-replacement-bmw-x5.html
- http://www.540i6.com/finalstagereplacement.html
This unit is unfortunately very often the source of other issues, the most annoying one would be draining the battery so you can not start the car. Definitely a recommended DIY job before proceeding to more expensive repairs.