When I put the fan speed on MAX I can feel some warmth coming from one of the rear vents but it's hardly blowing anything. I tried all possible combinations and only the windshield vents are working fine. Tomorrow I'll take off the cover under the wipers to see what's going on in there. I still believe that there must be something blocking the air..Check to see if adjusting the vent position and the air not coming out of the vents affects both driver's and passenger's sides or if it just affects one side.
You are looking in the wrong place...!