GM Service Manual Online
For 1990-2009 cars only

When you open either a front door or a rear door, the lamps inside your vehicle will go on. In addition, if your vehicle is equipped with remote keyless entry, the light will come on when the remote keyless entry UNLOCK button is pressed.

After you exit the vehicle and all of the doors have been closed, the lamps will stay on for an additional 20 seconds before fading off. The lamps will also fade to off when the key is inserted and turned to RUN or ACCESSORY, or the LOCK button on the remote keyless entry transmitter is pressed.