Bob R. 6/3/2010The fan is actuated by a temperature switch that is screwed into the side of the radiator. It also has a fuse and a relay in the fuse box in th engine compartment.
If you take off the connector and short the two prongs together with the ignition on, the fan should run. If it doesn't, it could be a bad electric motor, relay or fuse.
If you overheat this engine, the cylinder head will warp or crack and you will be out $4000+, so don't ignore this problem. I'd suggest you take the car to an independent shop that specializes in German cars, or contact the BMW Car Club for a reference. BMWCCA.org