The fan responds to a pulsetrain signal from the engine computer. If you have a true RMS digital multimeter, you can pull the connector off when the fan is running, and measure the voltage at the lightweight wire. If zero, the controller on the fan is bad.
Ed in San Jose '97 540i 6 speed aspensilber over aubergine leather. Build date 3/97. Golden Gate Chapter BMW CCA Nr 62319.