Radiator fan won't stop!

[netphreak]

One morning I found my car with an empty battery (Volvo 940, 1998)... I soon realized that the fan for cooling the radiator had been spinning my battery empty during the night. Well, it was winter, so I just unplugged the fan - problem temporarily solved. After a few days I plugged it in - and it worked just as it should again! Weird... But now the problem is back again

What could be wrong? Must be some elctrical error, or maybe the thermostat?