The A/C switch activates the fan =)
 

so if I had to guess I'm going to say the 3pin sender @ the bottom of the radiator is somewhat if not completely fubar.

Good news. I trust you can find the P/N. Check ATP. Might be one at a wreckers.

Kate says look in 959-50 No.8, 191 959 481C. Very common across the whole line 80,90,100, A6, A4,etc

The fan is supposed to come on with the A/C.