GM Service Manual Online
For 1990-2009 cars only

All of the doors will lock automatically when you move the shift lever out of PARK (P). This feature cannot be disabled.

If someone needs to get out of the vehicle while it is not in PARK (P), shift into PARK (P), or, have that person use the manual lever or the power door lock switch. When the door is closed again, it will lock when the vehicle exceeds 5 mph (8 km/h).

With automatic door locks, you can lock or unlock the doors at any time, either manually or by using the power door lock switches.