GM Service Manual Online
For 1990-2009 cars only

Your vehicle will automatically unlock all doors when the shift lever is moved into (P) Park for a vehicle with an automatic transmission, and when the ignition is turned off for a vehicle with a manual transmission.

If your vehicle has a Driver Information Center (DIC), the doors can be programmed to automatically unlock several ways for vehicles with an automatic transmission. See DIC Vehicle Personalizationfor more information.