Fixed my Final Stage Resistor yesterday in about 15 minutes with another 15 to remove and re-install. If you have a soldering iron and a little patience you might be able to fix it yourself before spending your money. read this:
http://www.xoutpost.com/bmw-sav-foru...r-fix-diy.html
This thread has pics of what the location of the FSR looks like :
http://www.xoutpost.com/bmw-sav-foru...sr-please.html
Instructions on how to remove the part is in the how to section
Front vents suddenly stopped working, hence no AC. I was looking at over $100 for the part at a dealership, but if you don't mind waiting for a cheaper one to be shipped it might be easier and worth it to simply replace it.
But to answer your question...I believe from yesterdays research, your problem is more than likely a faulty Final Stage Resistor.
hope this helps
Good luck
Roo