Chevrolet 2003 Silverado 1500 Pickup Owner's Manual page 170

The DRL system will come on when the following
conditions are met:
the ignition is on,
the exterior lamps control is in AUTO,
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the automatic transmission is not in PARK (P), and
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the light sensor determines it is daytime.
When the DRL are on, only your DRL lamps will be on.
The taillamps, sidemarker and other lamps won't be
on. The instrument panel won't be lit up either.
When it begins to get dark, the automatic headlamp
system will switch from DRL to the headlamps or the last
chosen headlamp setting that was used.
When you turn the headlamp switch off, the headlamps
will go off, and your DRL lamps will illuminate,
provided it is not dark outside.
To idle an automatic transmission equipped vehicle with
the DRL off, set the parking brake. Shifting the
transmission into PARK (P) will also allow you to idle
the vehicle with the DRL off. The DRL will stay off until
you release the parking brake or shift the transmission
out of PARK (P).
To idle a manual transmission equipped vehicle with the
DRL off, set the parking brake. The DRL will stay off
until you release the parking brake.
Fog Lamps
If your vehicle has fog lamps, use them for better vision
in foggy or misty conditions.
The fog lamp button is
located on the left side of
your instrument panel.
Your parking lamps and/or low-beam headlamps must
be on for your fog lamps to work.
Press the button to turn the fog lamps on. Press the
button again to turn them off. An indicator light will glow
in the button when the fog lamps are on.
Remember, fog lamps alone will not give off as much
light as your headlamps. Never use the fog lamps in the
dark without turning on the headlamps.
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