[Paul Bennett]

Best for the fan to know the temperature of radiator water. You can unscrew the radiator empty valve at the bottom and plumb in another place for the sensor. You could have your radiator man add a screw-in outlet at the tank (talk to your radiator man). Or you could fashion something in the hoses. I've seen that done but was leery. My former car which shall remain nameless had the first technique, sensor at bottom of the radiator. A second fan was activated if the a/c were turned on.