That's the classic
symptoms of a blend door problem. If you are lucky, your blend servo has failed. If you open the glove box all the way, until it hangs straight down, and look through the upper left corner of the opening at the heater, the little white box on top of the heater is the servo. Using a tiny screwdriver, pop the rear edge up off the clip pins. Using a larger, longer screwdriver, pop the front edge off the clip pins. Turn the ignition on, and the heater. Adjust the heat control from cold to hot, etc. The shaft should rotate about 90°, strongly, and reposition wherever you stop the heat knob. If nothing happens, ta da! You can fix your heater easily. Just make sure you line up the D shaped hole when installing the servo, you'll need a small mirror on a stick to look down into the heater. If, on the other hand, the servo works, you have the blend door problem. Use the same mirror to look at the D shaped hole in the top of the door shaft. If one or two sides of the D are broken away, you have a lot more work ahead of you. Just how much, I'll be finding out soon.