The aux fan has 3 wires, an always live positive, a ground, and a PWM signal wire from the DME.

The DME controls the fan taking input from the coolant temp sensor in the lower rad hose and the AC system pressure switch IIRC.

There is a diagram floating around here of the system as posted by Qsilver7 if you can find it.

A quick test to see if the fan is working is to put the AC on MAX. The fan should turn on.

Also, the fan self tests on cold starts, it should turn itself on for a few seconds IIRC.

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04 X5 3.0i auto
03 X5 4.6is