GMC 2010 Yukon/Yukon XL Owner's Manual page 235

For vehicles with fog lamps, the control is located next
to the exterior lamps control on the instrument panel, to
the left of the steering column.
The ignition must be in the ON/RUN position for the fog
lamps to come on.
Press to turn the fog lamps on or off.
A light will come on in the instrument panel cluster.
When the fog lamps are turned on, the parking lamps
automatically turn on.
When the headlamps are changed to high-beam, the
fog lamps also go off. When the high-beam headlamps
are turned off, the fog lamps will come on again.
Some localities have laws that require the headlamps to
be on along with the fog lamps.
If the vehicle has this feature, this button includes wiring
provisions for a dealer or a qualified service center to
install an auxiliary roof lamp.
When the wiring is connected to an auxiliary roof
mounted lamp, pressing the bottom of the button will
activate the lamp and illuminate an indicator light at the
bottom of this button. Pressing the top of the button will
turn off the roof mounted lamp and indicator.
This button is located on
the overhead console.
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