Chevrolet 1994 Astro Passenger Owner's Manual page 135

If your vehicle has rear air conditioning, setting it on LOW may enhance
front A/C performance by allowing trapped refrigerant in rear lines to
circulate.
The system will only send cooled air if the front system is on. It can still be
used to recirculate air, even if the front system is off.
Before using the rear air conditioning, open the windows to clear the vehicle
of hot air. Close the windows when using the system.
Rear Window Defogger
If your vehicle has this option the rear window will have lines running
across the glass. These lines heat your window.
For best results, clear the window of as much snow or ice as possible before
using the rear window defogger.
To turn on the rear
window defogger, find
the switch marked
REAR DEFOG on
your instrument panel
to the left of the
steering column. Press
the top of this switch
until the light in the
switch comes on, then
release it.
The rear window defogger will only work when the ignition switch is in the
RUN position.
You can turn the defogger off at any time by pressing the bottom of the
switch. The defogger will shut itself off after several minutes so that the
glass does not get too hot. If the window still isn't clear, turn the defogger
on again.
I
NOTICE:
Scraping the inside of your rear window could cut and damage
the defogger. Your warranty would not cover this damage. And
don't put decals there, you might have to scrape them off.
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