Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
Can you tell us what size engine and drive type are on that S10?
OK. I took a chance and uploaded the vacuum diagram for a 2003 S10 with the 4.3 V6 and 4WD (see attached graphic). Please let me know if yours is different.
Going by the symptoms you describe and knowing that the default air delivery position is Defrost, I would say the issue you're having is due to a bad mode switch in the control head/module - switch is integrated and not serviced separately.
Please let me know if you have any questions.