could be the blend door, it is a very common problem for first gens. I had to do mine recently and I got my heater to blow like a fireplace.
Another thing to look at though is the knobs themselves. At one point I thought I had a bad heater core but it turned out that on the back of my temperature knob, it was cracked and was not actually turning the switch that controls the heat but just moving around it. I would pull it off to make sure that it isn't cracked. Start with the simple things, right? lol