Defrost not working

I'm not very car savvy so bare with me. I have a 1990 Chevy 1500 that I recently purchased. I've since found a few quirks to it, but nothing too serious. Of those, the defrost doesn't seem to work. The heat turns on and blows very hot air without any problems, but the air ducts from the dash aren't blowing when the defrost is turned on.

I have no idea where to begin to fix this problem. Any suggestions?

Find out if the switches are vaccum switches or mechanically operated. Then you can know what you need to look for.

check around for a good mechanic, get a good referr too just to be sure