My problem (that is somewhat related) was in the summer, my defrost vent was always staying open with AC on, which kept frosting the windshield. Since I was not ready to pull the dash, I found a work around. While laying upside down in the foot well next to the accelerator pedal (so I could see the pod), I was able to wedge a large sized flat head screw driver between the pod and the control box which also compressed the rod down into the pod. This effectually closed the defrost flap sending all ac airflow through the center and side vents. I left the screwdriver in place and was able to screw back the foot well panel. This is not as good a solution as mentioned above since it will require removing the screw driver in the winter to get defrost heat again. Just for me, at least for now, I would prefer that to pulling the dash.

__________________
02 E320 Black (198K)
85 300D Blue (147K)
83 300D Blue (299K)
Plus 2 300D Parts Cars
05 ML350 Silver (165K)
02 ML320 White (207K)