HVAC Blower will only blow on floor ?

Hello, everyone. I own a 98' Chevy 2500K. My blower will only blow through the floor vents no matter what position I put the dash selector in. Any ideas would be helpful!! Thank you!!

That one has a electric mode door motor.
On front of heater housing someplace.
Check and see if it is moving or switching mode door from floor to defrost to vent position.
MT

The mode control door is located on the left side of the plenum around and above the tranny hump. On my '98 Sub the blower fan has to be running to activate the mode control door. In the center of the motor is a small round indicator that is notched so you can see if it's working. If it isn't it's either the motor, the control panel or wiring.