problem with a/c vent control 88 Silverado

This was posted in the incorrect forum (Uplander) so I moved it to this forum in hopes the poster could find some answers--->
I have recently purchased a 1988 chevy silverado 4x4. There seems to be a problem with the air conditioner vent control. It doesn't switch from the floor mode. If you try to turn the defrost on it blows on the floor as well. Anyone have any ideas what the problem might be.

Thanks for any and all help!!!!!!!!!