GM Service Manual Online
For 1990-2009 cars only

The lamps inside the vehicle turn on when any door is opened. These lamps will fade out after about 20 seconds after all of the doors have been closed or when the ignition is turned to ON/RUN. These lamps will also turn on when pressing the unlock symbol button or the horn symbol on the keyless entry system transmitter.

The lamps inside the vehicle will stay on for about 20 seconds after the key is removed from the ignition to provide an illuminated exit.