GM Service Manual Online
For 1990-2009 cars only

The vehicle is programmed so that, when the doors are closed, the ignition is on and the shift lever is moved out of (P) Park for automatic transmissions, or when vehicle speed becomes faster than 8 mph (13 km/h) for manual transmissions, both doors will lock.

Use the power door unlock switch to unlock the door when the vehicle is not in (P) Park. 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).