GMC Sierra Owners Manual
The dome lamps are located in the overhead console.
They come on when any door is opened and turn off after all the doors are closed.
Turn the instrument panel brightness knob located below the dome lamp override button, clockwise to the farthest position to manually turn on the dome lamps.
The dome lamps remain on until the knob is turned counterclockwise.
Dome Lamp Override
The dome lamp override button is located next to the exterior lamps control.
(Dome Off): Press the button in and the dome lamps remain off when a door is opened. Press the button again to return it to the extended position so that the dome lamps come on when a door is opened.