I have the same vehicle with the same problem. Inside the large black plastic heater/AC housing under the dash is a 'Mode Door' that controls which vent air exits (defrost, dash panel, dash & floor, or floor). The Mode Door is controlled by a vacuum Actuator near the gas pedal which has two vacuum hoses (brown & red). When a mode button is pressed on the dash, vacuum operates a rod on the actuator which moves the Mode Door. The rod is hooked into a hole on a small flange or tab of plastic under, and molded into, the Mode Door. It is not likely that the rod became unhooked because the rod can only be put into that hole when the actuator is hanging loose. The actuator is held in place with two hex head screws. What likely happened is that the plastic tab broke off. So when the blower fan is on the force of that air moves the Mode Door and the actuator rod is not controlling air direction.
Unfortunately to get access to the heater/AC housing is not possible without removing the windshield because of how four screws that hold the defroster ductwork to the dash were put in place. So to fix the problem you need to remove the lower dash panels, both on the left and right side, and the steel brackets that hold the ashtray/cupholder/storage box.
I drilled three 2" holes on the side of the heater housing to see the problem. Only one hole that large was needed, but at the time I didn't know exactly where the problem was located. I drilled a small hole in the remnant of the tab with the hope of wrapping a piece of wire abound the rod and through the hole to cause the actuator rod to operate the door, but that didn't work. The wire kept slipping of the rod as it moved.
I then made a small metal bracket with a hole, then attached it with super glue and a sheet metal screw to the underside of the Mode Door. With the actuator loose, I slipped the rod through my bracket hole, and reattached the actuator with the two hex head screws near the gas pedal. I tested it for a day or two and it worked. I then resealed the heater housing with the 2" discs of plastic and duct tape.
Images (Click to enlarge)
Tuesday, November 10th, 2015 AT 6:16 AM