Check the blend door actuator rod and especially the small plastic clip that joins the rod to the blend door. Most likely that plastic bit is broken...and the blend door won't close enough to give good defrost flow.
Okay if you look down at where the heater lets all the air out on the trans hump the blend door is to the right a bit in the black boxes. You have to take most of ducting out to be able to get to it. Its not bad and the part (if broken) is frickin 94 cent at the dealer. I had the same problem as you. Now I have great defrost and heat. /forums/images/graemlins/woot.gif /forums/images/graemlins/woot.gif /forums/images/graemlins/waytogo.gif /forums/images/graemlins/waytogo.gif /forums/images/graemlins/waytogo.gif
Sometimes it's as simple as a vacuum hose that came off. It is a bit nasty to track down the problems, hard to get at, hard to see etc etc. I much prefer the non AC heater controls and stuff...no vacuum involved and much simpler. The AC stuff used to give me fits and would regularly need my attention.