No air from the defroster

[crawdad]

I can't get any air to come out of the defroster ducts in my truck. It feels like all of the air is coming from the heater (floor) duct. I saw what looks like a vacuum port right next to the floor duct, but there was no vacuum hose connected to it. Could this be the problem? If so, where does the line come from? I couldn't find one anywhere. Course its so tight up under the dash, I probably would have missed it any way. What is the best way to make some room under there to get to anything? Thanks.