Air Distribution Venting Direction Problem

The majority of the Air when in any AC mod especialy in MAX AC on my 96 Jimmy comes out of the top Defroster Vent. I have replaced the distribution selector switch, checked and replaced all vacuum lines, and have even gone as far as putting a vacuum line directly from the engine to the distribution switch. All attempts fail to get the door that opens and closes the defroster center vent to detent. My short term solution has been to place rags under the defroster vent cover to block the major flow, and push more air out to the front center and side vents. I would like to check out the item (believe it's a vacuum modulator) that controls this operation. My question has anyone had to do this, anyone have a source/schematics on a good way to remove the dash top without taking more apart than necessary. Removing the radio and the selector assembly does not get this area exposed.