GM Service Manual Online
For 1990-2009 cars only

The exterior lamps turn off about 10 minutes after the ignition is turned to LOCK/OFF, if the parking lamps or headlamps have been left on. This protects against draining the battery. The battery saver does not work if the headlamps are turned on after the ignition is turned to LOCK/OFF.

To keep the lamps on for more than 10 minutes, turn the lamps back on with the exterior lamp control.