I had the exact same problem.Here what to look for.get down in the driver side floor board and look up under the dash where the a/c controls are radio all that stuff.There will be a litte metal Vacum can with some hose's running to it that is the vacum acuator it has an arm comming out of it with a hole in the end if you turn it to defrost and then back the a/c vent it will move out and in just in front of it it attaches to a nipple like piece sticking out.the hole at the end of the acuator arm goes over the little nipple sticking out which moves you from defrost to vents.Check it out.
