I have a 98 STS and the defroster won't work.
The car warms up nice and steady and the interior will warm up nicely as well.
The only place the heat comes out is the vents and the vent under the steering wheel.
The light on the control panel lights up correctly but no matter which defrost setting is selected, no air will blow out the windshield vents.
I can hear the blower running and even in defrost mode the interior will warm up, but I don't know where the air is coming out.
All I know is ice will NOT melt off the front windshield.

 

Sounds like your problem is the Mode door. Not sure about the newer models but the older ones had a vacuum motor that controls the mode of the defrost door. It would go either to heat or defrost. Youmight have to go into the dash and see if the door is stuck or check the lines that control the function.

 

Thats what I was thinking. A similar thing happened to my wifes car, fortunately it was under warranty. Mine is not.

Would you happen to know where I could find instructions for removing the dash? Would I just have to remove the top dash panel to gain access to this door?