Heating vents

2003 Chevy S10 pickup, 53,500 miles, 4.3 engine

When the heater is turned on in this truck, all the heat comes through the dash vents. The options are defrost, dash vents, or floor, but I am unable to switch to the different places. I like the airconditioning to come through the dash vents, but not the heat. Anyone know what has caused this, or how to fix it. It used to work properly. It seems to be stuck on the dash vents.