Defrost only through floor vents on 1990 Chevrolet 3500 Pickup

Why does the air in my truck just exit the floor vents when my defrost is on?

Asked by for the 1990 Chevrolet 3500 Pickup
Most commonly this would be caused by a vacuum fault. Make sure the vacuum supply to the a/c control is good and all the hoses under the dash are connected correctly.
Thanks for the reply. I kind of figured that. It seems all are connected properly, but one. It comes from the control panel and it's hanging loosely. It's got an orange stripe on it and is T'd at the other end with a connection on each of the ends. Both are "plugged" with a screw so I'm guessing the problem lies there. How do I find out where these go and how do I get to it? As you can tell I'm a novice at this, but I'll try to be a quick study.
1 more answer
its possible somthing has dropped into the defrost vents and has lodged itself at the flap inside duct that controls the air direction. i had this problem with a ford pickup. i took apart the duct work and there was a pen keeping the flap for the air in open. took it out put it back together ....problem solved