1999 Chevrolet S-10

Heater problem
1999 Chevy S-10 4 cyl Two Wheel Drive Manual 135000 miles

air blows thru dash vents only. Will not blow thru defrost or floor vents
October 25, 2009.

You are looking for a vacuum leak. The vent doors are actuated by vacuum. Check under the dash and the hood for leaks. Many times, you will hear a hissing sound where it is leaking from.

Let me know what you find.