Climate control Fan problem
 

I have an 87 5000S Quattro and the Climate control fan will come on when I start the car but then gose off. If I push the any of the fan speed buttons nothing happens, also nothing happens with the fan when I adjust the temp. if the sensor in the dash is dirty will that cause the signal to get screwed up? if so is there away I can pop the little cover off the dash to clean it? Or is there somthing alltegether different I should be looking for? there are no codes showing on the led display of the control unit. Any help would be appriciated.

Mike[X(]