GMC 2003 C4C042 Owner's Manual page 134

When you turn the headlamps off, the regular lamps will
-
go off, and your low beam headlamps will change to the
reduced brightness of DRL.
To idle your vehicle with the DRL off, set the parking
brake. The DRL will stay off until you release the
parking brake.
As with any vehicle, you should turn on the regular
headlamp system when you need it.
When it begins to get dark,
your DRL indicator light is
a reminder to turn the
headlamps on. The other
lamps that come on with
your headlamps, will also
come on.
Marker Lamp Defeat Switch
If your vehicle has this feature, you can manually blink
the marker and clearance lamps.
Press and hold the bottom of the switch to turn off
the marker and clearance lamps. When you release
the switch, the marker and clearance lamps will come
back on.
The marker lamp
defeat switch is located
in the instrument panel
switchbank.
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