GM Service Manual Online
For 1990-2009 cars only

The Central Door Unlock feature allows the operator to lock all of the doors from outside the vehicle by using the door lock key in the drivers door lock. When the key is inserted into the drivers door and held to the Unlock position for approximately two seconds, or turned to the Unlock position twice in approximately three seconds, the Key Cylinder Unlock Switch that is mounted internally closes, and all doors will unlock.