Does your a/c work sometimes or never? Are you having any other electrical problems?

Disconnect the aux fan, start the car, and see if the compressor ever turns on. On e39's a shorted aux fan can cause IHKA problems. A used IHKA (if you still need one) looks to be around $250 on ebay. For e39's, if you buy the same one (same buttons) it doesn't have to be coded to the car ($$$). Not sure about that for e53's so do your own research.