Heater control?

[Atommik]

I've got a 1999 Suburban 4X4 with the 5.7 engine. The heater control that changes/blends air flow from defrost to middle vents to floor, only will change from middle vents to defrost. No air flow from the floor vents. The fan is working, heat is good, but almost no air at all from the lower vents. It just keeps coming out of the defrost vents when it is turned to floor. Is the blend door, vacuum controlled or electronic? Or is it more likely the control knob since the blend door does change from defrost to middle vents? Any ideas on how to test it?