an average set up is a sensor plug on top of rad or leaving engine after thermostat this sensor is called a thermistor and if you wire it through a relay for the fan and use the thermistor as an earth supply to the relay then your spark will be much reduced and therefore your thermistor will last years instead of days doing the heavy slogging under the full power required to run the fan being run through it .
you can buy thermistors of many temp ranges and threads and you do not need to use a bmw if not available to you also the fan is not required above 15mph as the wind takes over --no need to have the fan running all the time --check the temp required and off you go . bugs