1997 Ford Windstar 3.0 197,000 miles. The heater is not working at all. The blower, and AC work, but when I turn the temperature control knob to the hot position, the air stays cold. Both lines coming through the fire wall to the heater core are hot, and the heat guage is reading at the half way mark. I have plenty of coolant. Might this be a control problem in the switch? Where should I look? Thanks a bunch. Just another poor shadetree mech.