GM Service Manual Online
For 1990-2009 cars only

The lamps inside the vehicle come on when any door is opened. These lamps fade out about 20 seconds after all of the doors have been closed or when the ignition is turned to ON. They also go on when you press the unlock symbol button or the horn symbol on the Remote Keyless Entry (RKE) system transmitter.

The lamps inside the vehicle stay on for about 20 seconds after the key is removed from the ignition to provide light as you exit.