the common mistake for people to make, is to fit the fan switch on the radiator inlet (i.e. top hose). think about it, whenever the thermostat opens, the water will always come through at a certain temperature. so the fans will come on, whether the car is moving and cooling the water in the radiator or not.

if you put it at the bottom of the rad, or in the bottom hose (outlet from the radiator), the fans will only come on if the car is stationary or there is not enough air flow to cool the water when it passes through the radiator.