Hey guys...did fix my problem and NO you do not have to replace the HVAC system. If you are getting air out of every area but one....meaning: face, floor, or defrost, then what I discovered should fix the problems. When I would change to defrost from face or floor, I would hear the vents to those locations close and hear the air change over to defrost, but no air would come out. My problem was that one of the small little motors of the defrost actuator came loose from its connection and was not opening the defrost flaps. The actuators of all the HVAC are behind the glove box towards the center console, or the center of the dash board. I think there are 3 actuators, one for each area, and 2 motors per actuator. The motors are the little round things that look a little bigger than a bottle cap. They clip into the actuator and have little motors in them with little teethed wheels. The motors spin the and the correct flaps are supposed to open and close. Check yours out and see if one of the motors are loose on the actuator, mine was and just by clipping it back in the housing the defrost started working.