Air blows through floor and defrost but will not blow through dash vents

If you have vacuum-operated mode door motors, you should be able to hear them banging and clanking when they move. That will prove you have a good vacuum source feeding the switch assembly. If you hear something change when you change the selection, but no change occurs where the air blows from, the best suspect is a broken mode door. The motor could have a leaking diaphragm too, but you'll usually hear the hissing. That is not as common as a broken door.