A Solution for Defrost/AC/Vent Issues
After researching this issue I found the following and hope it will help all, note my truck is a Ford F-250 Diesel “2002”.

If you lose air to your vent system except the defrost try the following solutions. But first remember this, the default to the climate control is defrost, “Not Off”…. Per Ford..

So with that.. Ford truck vent systems run on two processes Electric and Vacuum.. You will need to find which one is going bad or has issues.

First check under the passenger dash board foot area to see your passengers kicked any vacuum tubes off.. If so put them on and test the vent system.. (This is Vacuum) Also with this follow the tubes to make sure they are not cracked or damaged there will be issues if they are.
Second if 1st did not work. Open the hood.. Stand in front of your truck with your left hand to your passenger side of truck. At the corner of the firewall and the wheel well you will see vent accessories, wiring and tubes. You will also see what looks like a fuse box my truck has a white cap on it make sure this is tight and did not come lose. (This is Electric) if lose (my truck was from the diesel) it will cause your system to go to default air.. To fix it simply push or tap on the cap and check the wiring and tubes in this area..

Good Luck and hope this will help somebody..
I am not a tech so all my advice is pure Diesel Ford Truck Nut!! And I have a XPower chip email me if you are interested http://www.bd-power.com/ford/xpower_video.php

-jimS