I have a 1992 S10 Blazer. It is 4wheel drive, four door Tahoe version. It has the 4.3 TBI with 354,000 miles on it.
The air seems to blow through the defrost all the time. Air does go through the heater and vents but still comes through the defrost. I looked at the heater assembly down by the floor and it looks like there is a hole in the side where their used to be a screww or something. If I reach in and grab the little arm inside that moves back and forth and stick a nail through the hole and keep the arm from moving back, then I have full heat with very little defrost. I can move to air and it does change from heat to air.
Is there a bracket or something that has fallen off? My wife and kids drive it so they may have found something on the floorboard and tossed it.
I also have a vacumn leak on the controller but that as my fault not geting it back together and aligned properly. I will redo that after fixing the blower switch. It melts about once a year and I have to clean up the contacts and put the switch back on.