Mustang 2.3 cooling problems

[Big-T]

The car is a 1989 Mustang 2.3 four-cylinder with automatic. I have some cooling problems and have 2 questions:

1. Does anyone know where the temperature sensing switch for the radiator fan is located? I tried following the wires but got lost.

2. The engine never seems to get up to proper operating temperature. I have replaced the radiator (it was leaking), the main hoses (to the radiator), and the thermostat (3 times). The only time it will reach proper temperature is when it is standing still (no airflow through radiator) while idling for a while. As soon as I start driving again the temp gauge falls down to the very lowest end of the gauge range within a few minutes. If I idle long enough while standing still it will eventually reach max temperature on the gauge, since the radiator fan is not working (see question 1. above). It falls down to bottom of the gauge quickly when I start moving though, regardless.

Any advice is greatly appreciated.

Thanks!

Big-T