1992 chev pickup air vents?

is there a air shutter vent inside the box that controls where the air goes, if so how to i get to it, air comes out of the floor and dash vents but not the defrost vents, check all the cables they seem to work when i but it on deforst it seems plug up, when i but it on the floor in the cold postion it blows fine when i but it on the hot position it blows on the floor and dash vents but seems plugged up and the fan rattles thanks for any help and any diagram