This morning at 5am I'm driving to work and my defrost isnt working, making it very hard to see out of the windshield. The selector seems to work cause I can turn it off, but its stuck with pushing air out on the floor. No other position works. I'm thinking a vacuum line might have gone T.U. Its a 78 Jimmy with A/C. Any ideas?? Its a bitch to drive in the snow when you cant see!