stuck in defrost

Today I started my truck (99 xlt 4*4 supercab) and although it heated up I could only get heat to come from the defrost mode. The hot/cold knob worked fine and the fan knob worked fine but regarless of where the vent knob was it only blew out at the windsheild. It was very cold outside about minus 35 c (I don't know the F conversion), is it possible that what ever controls the vent selector is just frozen and it will start to work better when the outside temp increases or is it not going to be that simple?
Thanks.

The blend door actuator could be stuck or inop. Work the selector knob with the key on & listen for where it's located under the dash. Hope that helps!
G/luck
Joel

When I operate the control with the truck off there is no sounds. I do remember when it was working right that if I did the same thing I would hear a vacuum sound.

Put a vacuum gauge on the vacuum motor that drives the blend door, does the amount of vacuum change when the controls are worked? When a climate control system loses vacuum, it defaults to full defrost so you can still clear your windshield. Check for a vacuum hose that is loose or broken causing you to lose vacuum.

the part you need to check is behind the glove box.blower motor is to the right w/ fresh air door and blend door is to the left. you cant see it but the linkage goes in on the top. at least you can see if it's moving. if i find the post with pic's i'll post it.

i just rembered that i had the same prob w/ mine last year. there was a vacume hose on passenger side from a vacume canister to blowwer motor that got melted somehow. check vacume lines under hood from blower motor to canister to engine.can't find the link to show pics of divertor door behind glove box

here is the pic'shttp://home.earthlink.net/~kandc1/ford/blenddoorfix.html