GM-GMC: heater fan, vacuum leak, floor vents

Question
I have a 1994 S-10 4x4 pickup.  Recently the heater/AC fan will only blow as if it is set to the blend setting, no matter if you set it to defrost or any other setting.  So it always blows split, through the defrost vent and the floor vents.  Do you have any idea what might cause this?

Answer
Generally if there is a fault in the system it defaults to the defrost setting.  With the engine running listen for a vacuum leak somewhere. you should be able to hear it, it will be a small whistle like noise.  Short of that, remove the hvac control head and with the engine running check to see if there is supply vacuum. I would suspect you have a control head problem.
Hope this helps