GM Service Manual Online
For 1990-2009 cars only

When you open any door, the lamps inside of your vehicle will go on. These lamps will fade out 20 seconds after the last door is closed, or when the ignition is turned on after all doors have been closed. These lamps will also go on when you press the trunk release button, the unlock button or the panic button on the remote keyless entry transmitter.

The lamps inside of your vehicle will turn on for about 10 seconds after your key is removed from the ignition to provide an illuminated exit.