GMC 2002 Sierra - Manual (Page 147)

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Exterior Lamps
The control on the driver’s side of your instrument panel
operates the exterior lamps.
Turn the control clockwise to operate the lamps.
The exterior lamp switch has three positions:
(OFF): Turning the switch to this position turns off
all lamps, except the Daytime Running Lamps (DRL).
It also puts the system into automatic headlamp mode.
(Parking Lamps): Turning the control to this
position turns on the parking lamps, together with
the following:
D Sidemarker Lamps
D Taillamps
D License Plate Lamps
D Instrument Panel Lights
D Roof Marker Lamps (If Equipped)
(Headlamps): Turning the switch to this position
turns on the headlamps, together with the previously
listed lamps and lights.
You can switch your headlamps from high to low - beam
by pulling on the turn signal/high - beam lever.
A circuit breaker protects your headlamps. If you have
an electrical overload, your headlamps will flicker on
and off. Have your headlamp wiring checked right away
if this happens.