cheggie

My 1996 Civic recently developed a problem with the windshield defroster. I have limited or no airflow from the vents at the top of the dash, but air flowing from the outer dash vents. I have flow from the floor vents when selected. I have checked the mode servo, it appears to work perfectly. I can see it cycle as I manipulate the 5 mode buttons, no broken linkages. I'm wondering which linkage routes air to the defrost vents, maybe a broken mode door in the case? Anyone have this problem - no defrost with a good mode servo?

Thanks for any insight,

Chris