Hello, I am having the same issue on my 2005 X5 (3.0D - Australian Right hand drive).

The Aux fan does half a turn when the AC is turned on but then never re-engages. Unplugging the sensor on the A/C HOT line (as it's been suggested on a separate thread here), turned the AC off (can tell by drop in engine revs and that 'AC was just turned off but air still blowing' smell in the car), it did not make the AUX fan go full speed.

I have also tried to disconnect a lower coolant sensor (which did nothing to affect the AC)

All of the above was done when car was at proper running temp.