I've never checked or looked at a circuit diagram, but I would expect the radiator fan to run off a break relay, so the temperature switch turns the fan off when the engine coolant is cold rather than turning it on when the temperature rises.
Disconnect the connector from the fan switch (the sensor that points towards the bulkhead on the thermostat housing) and short out the pins with a jumper wire, if the fan stops, then the switch will most likely need to be replaced.