1978 GMC defroster problem?

I have a 1978 GMC 1/2 ton pickup. There is something wrong with the defroster. When I turn on the defrost it just barely blows. When I change to the floor, it blows like crazy, and when I put it on the vent it blows hard there too. I can hear the blower going full blast when it goes to defrost, but not much air coming out there. A little air comes out the floor and the vent then too. I have dropped the glovebox down to watch the levers work, they do. I have unhooked the lever and did it by hand and still very little going to defrost. Any ideas??