GM Service Manual Online
For 1990-2009 cars only

Your vehicle is programmed so that, when the doors are closed, the ignition is on and the shift lever is moved out of P (Park), all the doors will lock.

If someone needs to get out while the vehicle is not in P (Park), have the person use the power door unlock switch. When the door is closed again, the doors will lock either when your foot is removed from the brake or the vehicle speed becomes faster than 8 mph (13 km/h).