air conditioner vent control

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!!!!!!!!!

ummmm... this is the chevy uplander forum. Please try the Silverado forum. Erik

I moved this thread over to the C/K forum where there is a better chance to find any answers.