97 S10 heater control quit working

I can't turn it off of AC. The knob turns but the air still comes out of the vent only. I can get heat out of it, I just can't get it to blow anywhere but out of the vents. Now that the weather is getting colder, I really need my defrosters. I checked all the vacuum lines under the hood and they all look ok. anybody know what else to check?

Check the vacuum lines to your 4WD if you have it. A split vac. line that leaks will lower the vacuum and it's ability to do work. It also may be a diverter valve problem...check for a hissing sound & I'll check the manual.