Have a Car Question? Ask a Mechanic Online.
Why not wire it up to the ignition switch so when the ignition comes on the fan also comes on. Then when you shut the key off the fan will go off. The only down fall is it will take a little longer for the car to heat up but wont cause any other problems. As far as a sensor to do this on newer cars they do have the sensor, but it is controlled by a computer for what your doing that would not be practical.