Isuzu 2000 Trooper Workshop Manual page 35

1A–6 HEATING, VENTILATION AND AIR CONDITIONING (HVAC)
Ventilation
Set "AIR SOURCE SELECT LEVER" to "FRESH"
position and turn on the blower fan. Heating can be done
in this lever position, sending in fresh air from outside.
The blower fan also serves to deliver fresh outside air to
the vehicle interior to assure adequate ventilation.
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