I understand what Wes is saying and it makes sense, however, IMO, the temp reference point for the fan control is not as important as everyone is making it sound, as long as the sensor for the control is matched for that reference point or your control is adjustable.
The water coming out of the engine effects the temp of the radiator so a sensor in the radiator will work fine. Because the water coming out of the engine effects temp of the radiator, a sensor in the area "below" the thermostat, in the intake manifold, will work fine also. Because the water that circulates in the head effects the overall temp of the water in the cooling system, a sensor in the head will work fine, however, the temp rise in the head happens much quicker (close to combustion chamber) so you would need to keep this in mind when designing your fan control system or your fans would come on very early.
Because the water coming out of the engine does not effect the temp of the tailgate, a temp reference point for the fan control at the tailgate will not work fine and should be discouraged.
