02 Taurus SEL No air flow to front defroster

I am suffering from a very weak air flow to the windshield through the defroster vents. All other HVAC functions work well, i.e., good heat through floor vents and dash vents. AC is good. Blower motor runs strong. I just can't get much air onto the windshield on both driver and passenger sides. I removed the radio center panel thinking I might have a vacuum motor failure or a vacuum leak, but everything seems fine. When I activated the defroster button, and observed the vacuum motor drive the arm to position the floor, dash and defroster vent door selector. It seemed to move through its range of motion OK, although it is difficult to get a good view of things in there! Does anyone have any suggestions what is wrong and how I can fix this vexing problem? Thank you very much in advance.