Heater control switch?

I have a 1999 Lt Suburban 4x4. My heater control switch(thing that switches from defrost to vents) is not right. Which ever setting it's on...it always blows up on the windshield(defrost). I know this is probably a real easy fix for most of the REAL mechanics on here. Any ideas? The fan also has all speeds working....

"99 was a changeover year body wise. So keep in mind I'm going to be talking about the "old" body style 96-98 type. The mode control door is driven by a motor that gets it's signal from the selector switch. So besides a wiring problem it could be the selector switch, the motor or the door is stuck. The motor is on the left end of the heater plenum to the left of the tranny hump on the inside firewall and is very easy to remove. Once you remove it use needle nose pliers on the door shaft to check for free door movement. Check the motor by selecting positions on the switch(on my '98 Sub the blower fan has to be in any "on" position for the mode control motor to run). The motor looks like a small upsidedown cabin with the roof peak at the bottom. There is also another motor that runs the temp door. It looks just the same but is on the aft side of the plenum so don't confuse the two. Hope this helps................Steve