I think the previous owner may have done what's known as the "fan delete mod" to your car. Check the link below. In the diagram is the fan I'm refering to. It sits behind the radiator.
http://bmwfans.info/parts/catalog/E3..._fan_coupling/
This is the electric fan I've been refering too:
http://bmwfans.info/parts/catalog/E3..._fan_coupling/