here is the crash course on IHKA
First of all, change the microfilter inside the car. That is probably stuck and has been change a looong long time ago.
http://www.bimmerboard.com/forums/posts/54597
All components and functions
http://bmwe32.student.utwente.nl/johan/ihka/ihka.html
Here you can see also some of the other possible proiblem areas such as the IHKA control module, the socalled sword, the IHKA control module.
The fan blower is located in front of the heaterbox. It's speed is controlled by the so called sword. The sword is located inbetween the evaperator and the control unit. Erratic fan behaviour is often caused by the sword.
But first start the easy things: replace microfilter, use electronic contact spray on the switches of the control panel (can be disassembled also, deneding on skill), the connectors behind the control panel, the connectors on each side of the control module, driver and pass side.
Sword repair:
http://www.bmwe34.net/E34main/Mainte...ical/Sword.htm
http://bimmer.roadfly.com/bmw/forums...1020502&page=4
Bookmark my website below and this is also included there
http://www.e38.org/e32/