Usually it's triggered by a dedicated sensor that comes with the fan kit, your temperature sensor / gauge uses a voltage range and is generally unsuitable for triggering a fan. Modern cars the fan is controlled by the ECU on the value of the temperature sensor (often different to the gauge's sensor).
So your options as I see it are:
- Manually control the trigger to the fan with a switch in the cabin like you would for say driving lights etc.
- Buy a sensor kit that inserts into the radiator hose (mostly?) and use this to trigger the fan. (Davies Craig do one I think).
- Wire up the fan permanently controlled from the ignition so it's always on (not as bad as you think,the standard fan is always going but admittedly varies with rpm)
You mention it as an auxiliary fan I take it this means you are retaining the mechanical fan? If so it probably means you don't want it on permanently, so a switch or sensor.
