GM Service Manual Online
For 1990-2009 cars only

The automatic transmission shift lock control system prevents the automatic transmission from being shifted out of PARK unless you depress the brake pedal. When the brake pedal depresses, the automatic transmission shift lock control solenoid releases a lock paw. This enables the movement of the shift linkage and the operation of the shift lever.

When service operations are required, overriding the automatic transmission shift lock control is possible. In order to override the automatic transmission shift lock control, turn the ignition switch to OFF (not LOCK). Remove the battery voltage from the solenoid.