GM Service Manual Online
For 1990-2009 cars only

Move the lever to the following positions:

(Off): Turns the lamp off, even when a door is opened.

(Door): Turns the lamp on whenever a door is opened. The dome lamp and mirror reading lamps will turn on if the lever is in the door position and the instrument panel brightness control is turned to the brightest setting. See Instrument Panel Brightness.

(On): Turns on the dome lamp.