1993 Buick Roadmaster Owner's Manual - Page 133

Defrost This setting will direct most of the air flow toward the windshield. Use this when or there is fog ice on the windshield. Rear Window Defogger (Option) if is Defrost will work better any ice and snow cleared from the hood and air inlet area between hood the the and the windshield. or Blower speed will be controlled automatically, you can choose another speed by pushing HI or LO the buttons. If you havethe heated outside rearview mirrors option, is the mirrors will be warmed when the rear defogger on. 131