The transistor symbol in the wiring represents some form of signal processing or a driver circuit - it's common in BMW wiring diagrams to represent complex circuits with a transistor symbol.

The aux. fan is controlled by the DME/DDE via pulse width modulation, on the third (smaller) wire. The fan driver in the hub of the fan (in the later versions - it's in a separate box on the fan frame on earlier models) uses the PWM signal to modulate the fixed +VE input voltage and control the fan speed.

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Wayne
2005 BMW X5 3.0d (b 02/05)
2001 BMW F650GS Dakar (b 06/01)