I have a 1995 Lumina that when you are using the Heater or the AC you can not change what vents you want to operate. Its stuck on the defrost vents. Anyone have an idea what controls these (is it vacuum?, electrical?) I haven't a clue.

I'm not sure about 1995 Lumina...older cars,back in the 70's used vacuum...Newer ones..2000 and up use electric motors for the vent/defrost/heat controls.You will have to tear it apart anyways to get at it..then you will know when you get down to it.There is a door that controls where the air blows.

Check the vac line going under the intake to the vacuum ball in the bumper. That'll cause the issues.

If you are still wondering, I have a picture that could show you how the system works...not good for locations though. It would be logical to check all vacuum sources, actuators, and solenoids in this situation to properly detect the problem.