GM Service Manual Online
For 1990-2009 cars only

The lamps inside the vehicle will go 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/RUN. These lamps will also go on when pressing the trunk release, unlock symbol, or the horn symbol button on the Remote Keyless Entry System (RKE) Transmitter.

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