Chevrolet Repair: 1999 Blazer 4.3 heat problem, vacuum problem, vacuum line

Question
James, appreciate the help.  Just went for a ride in my Blazer. First day of cool weather here so I turned on the heat. The air got warm so no problem there, but the heat comes out the AC ducts. Switch seems to be working, as AC compressor comes on at AC and Defroster settings, but the air does not change-still comes out the vents. I checked fuses 9 and 21. Both OK. Does this sound like a vac problem? Where should I look. Any assistance appreciated. Thanks  

Answer
Yes it is a vacuum problem. There is a vacuum line that is attached to a softball size looking ball that is probably disconnected.It is located in the engine compartment  over on the left side towards the back.
They may have changed the ball to a smaller size or even deleted it all together in which case you may have to go under the passengers side dash.
If you cant find the problem there, you may have something in the control door box itself blocking the door.