I have a 96 chevy blazer, im having problems with the heater it only blows hot air through the defrost vents, i try switching to the floor or other settings and nothing happens, what can the problem be?

 

there is a door inside the heater box on the drivers side , when this door is inoperative it defaults to blowing out the defrost....check its actuator or the door itself.....my door shaft broke and I fixed it by using a dremel to cut into the box and remove the door . Do a search on this site and all the info should come up...it has happened to many members.

 

You could also have a vacuum leak. Which put it in default settings, defrost