GMC 1997 Safari Owner's Manual page 151

Rear
Air Conditioning System (Option)
You may have rear air conditioning. The rear air
conditioning switch
is
located to the right of the heater
controls. Turn the switch toward HIGH for higher
airflow in the rear area. Turn the switch
to
the OFF
position to turn the system off.
Air Conditioning
Before using your vehicle's air conditioning, open the
windows to clear the vehicle of hot air. This reduces the
time it takes for your vehicle to cool down. Then keep
your windows closed for the air conditioner to work at
its best.
You can use MAX A/C with the temperature knob in the
blue area when it's really hot outside and you need to
cool the inside air quickly. MAX A/C lets in only a little
air from the outside.
If you first use MAX A/C, you can then use A/C with
the temperature knob in the blue area, as soon as the
vehicle has cooled down, so outside air will be going
through your vehicle.