That is the small fan behind the IHKA control panel below the radio, which blows the interior air thru a funnel where a temp sensor is located and that is very important for the good operation of the heating and cooling system.
Worth to have a look inside and clean it, look at this dirty stuff, many pics
http://www.bimmerboard.com/forums/posts/425554

I forgot to add:
there are in total 5 temp sensors which are important for the IHKA heating and cooling system.
Pics where they are and test data here
http://bmwe32.masscom.net/johan/ihka/ihka.html

This does not include the temp sensor which is in the front grill, as that is just for the OBC but has no relation to IHKA