2001 E-450 no defrost

Hey guys.
What I have here is an E450 cutaway bus with a 7.3. It came in today with the defroster not working. It has vent/heat/a-c, and the fan works at all speeds, but nothing coming out of the defroster vents. These vans are set up so that if anything (usually vacuum) goes wrong, it'll go to defrost as a default. It couldn't be driven as we had snow today, and everything kept freezing up on the windshield. Any help would be appreciated.
Thanks

Ended up taking the steering column, and the whole dash assy out, only to find somebody got a pen cap jammed in there.:banghead: It jammed the door that directs the airflow to the defroster. Needless to say I was less than happy. Turns out one of the passengers in notorious for playing with pens, and such. He likes to stick them into the dash vent openings. I got written up for giving the driver hell, but at least I know I won't be doing that job again!! :rolleyes:

MIKE