These are controlled by a PWM circuit, there is no cable to regulate the temperature.
Easy way to test that the control circuit is working -
Get a 12v 5w bulb and couple bits of wire (I use an old mk4 Escort side repeater bulb/holder)
Disconnect heater control valve connector and connect the bulb to it.
With engine running, and heater hot/cold in varied positions the bulb will either
Stay off in one position
Be on all the time in the opposite position.
In the middle position it will flash about once a second.
http://www.fordpartsuk.com/shop/ford..._valve_531.htm