Take another look at the valve, late car should have the plastic heater control valve like this on the left. I think you would see the lever move when you adjust the heat. When the engine is warmed up is it warm on both sides, it is open, if cold on one side it is close.
http://www.944s2.com/images/heatervalve.jpg
Look at the diagram on Clarkes link above for location of the solenoid valve, it is under the dash, right side...