In earlier models of the S-10 the vent position was controlled by vacuum
from the motor, and air coming out from the heater ducts under the dash only
was a symptom of a leaky vacuum line or fitting. However, in 1996 the air
valves (flapper valves) were controlled by electic servos that moved the
flap to send the air to different vents. It's not uncommon that after
nearly a decade you would build up enough dirt and gunk on the lip of the
flapper valve to cause it to stick in one position, or that a servo would
die or get stuck. You're going to have to have someone open up the air box
and look at the valves and servos.
Cheers - Jonathan
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