GMC Sierra Owners Manual
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.
(Fog Lamps): 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 with the fog lamps.