Buick 1993 Roadmaster Owner's Manual page 133

Defrost
Rear Window Defogger (Option)
This setting will direct most
of the air flow toward the
windshield. Use this when
there is fog or ice on the
windshield.
Defrost will work better if any ice and snow is cleared
from the hood and the air inlet area between the hood
and the windshield.
Blower speed will be controlled automatically, or you
can choose another speed by pushing the HI or LO
buttons.
If you have the heated outside rearview mirrors option,
the mirrors will be warmed when the rear defogger is
on.
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