1994 GMC C1500 no defrost

I have actually run across this problem in the field before. On the drivers side of the heater case is a mode control actuator, which is basically an electric motor. It has a little spring loaded lever hooked up to it, and that lever is very prone to seizing, and once it seizes, the climate control system usually gets stuck on floor outlet only. And if it seizes, you may have to replace the actuator as well. The fix is to remove the actuator, and use some kind of penetrating oil on the lever, after you spray the lever, work it back and forth several times until it moves with very little pressure. You should also work the spring loaded portion until it is free as well. If the lever moves fine with the actuator removed, try moving it through different positions, and make a note of where the air comes out of the vents.I believe with the lever in the full up position you should have your defrost back. By the way the actuator is a bugger to replace it has only 3 screws, but they are in a very difficult position to remove.