GMC C-Series 2001 Owner's Manual page 94

Marker and Clearance Lamps Switch
(If Equipped)
If your vehicle doesn't have a radio, the switch is
located just above the storage area. Use the switch to
operate your clearance and marker lamps. The marker
lamps are illuminated when the indicator light is on.
2-50
Your vehicle will have this
switch only if it is designed
to pull a trailer. If it has this
switch, you'll find it located
on the upper right side of
your instrument panel, just
above the radio.
Your headlamps switch will not operate these lamps.
You can use the switch to manually blink the marker and
clearance lamps by simply turning it on and off.