Basically the heater control panel (the hot/cold temp control) is a potentiometer that sends a PWM (pulse,width,modulated) signal to the heater control valve which has a motor on one of the connections.

Depending on the postion of the control, the motor is open/closed to regulate the flow of coolant to the heater matrix.

This can simply be tested by connecting a bulb into the HCV connector, with the car running, in the middle position between hot/cold, the bulb will flash about once a second, and with it either hot or all the way cold it will either stay on, or be off.

edit - engine has to be running!