1990 Bronco Floor Vent doesn't work?

Winter's coming up and I haven't fixed this yet. Not sure what I'm even looking for at this point. The vent and defrost settings both work fine, however the a/c and floor vents do not work. I'm not concerned about the a/c since I have windows, however I am concerned about my feet freezing again this winter. If someone could give me a list of possible problems/solutions or an idea of what I want to look for that would be great. The service manual doesn't cover this one.