My guess is either the heater core is clogged, or the blend air door is stuck in the cool position. The "door" is really a flap which diverts air past the heater core when its set to the cold side, or through it when its set to the warm side.
If you have a climate control, its possible that a small vaccuum hose has come loose or that one of the control diaphragms is not working. I don't know of a simple way to check it without dissassembling the dash.
On the earlier models w/o climate control, there is no actuator or vaccuum line for that setting; the door is moved by a cable, so if the slider moves real easy, usually somethings wrong with the cable. Either its broken, or whatever it attaches to is broken.
One thing you can try is with start the car and turn everything which can make noise off, and the fan on low. Then turn the temp knob to cold and then hot and with your ear at the vent, listen for a wheeze sound and a faint thud when the door reaches the stops. You should hear stuff moving around and some wheezes whenever you switch around from defrost, vent, max A/C, and norm A/C. You may have to get under the pass side dash to hear it.
And of course after reading your message again, I'd suggest you check the vaccuum hoses under the hood. The hose for the A/C controls is very small and runs from behind the intake plenum through the firewall where the A/C lines go through.
Hope this helps.