1996 220C - When I turn on the climate control to defrost (HI) it blows hot onto the winshield and side vents. If I put on auto or any of the other settings.. heat, heat - vent, it blows cold. It even blows cold through the defrost vents when I put the climate control direction to up only.

Not sure where to start looking but obviously there's a switch or valve malfunctioning. Any one have an idea what's happening and how to fix?