Heater won't switch between Floor/Vents/Defrost

So my heater stopped switching between Floor/Vents/Defrost. No matter what setting I put the dial on, it always blows out the vents. The heater works good and blows hot, just can get it to switch to the floor or the defrost.

Anybody else experience this? I'm not sure where to start diagnosing this issue. Thanks!

http://www.explorerforum.com/forums/showthread.php?t=369287


search term to use

defrost vacuum

Then pick from the threads in the 3rd gen sub forums for answers.