Redwood

After the car is warm, the heater in my 88SE works sparatically, at best. The blower works great and blows hard and the various vents can be switched to without a problem. The issue is that the air coming out remains a constant temp, despite the position of the temp thingie (cold or warm). Initially, I figured that something in there must be stuck but then I made a strange discovery. When I push in the button to select the vents (such as defrost, feet, etc.) the air suddenly becomes either hot or cool depending on the position of the temp thingie.

No idea what to do here and I'm freezing my *** off. Please help. Thanks.