Volvo VNL Operator's Manual page 114

Instruments and Controls 109
Heated Mirror Switch (option)
The vehicle can be equipped with heated
mirrors. The switch turns on an electric
heater element that is a part of the mirror
glass. The heater is self regulating and will
keep a steady temperature after reaching
operating temperature. Both top and bottom
mirrors are heated.
Turning the switch on activates a telltale in
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the instrument cluster. The telltale will stay
on until the switch is turned off.