BTSI Solenoid
WARNING: This page is about a different car, the 2005 Saturn L300. However, it is still accessible from the selected car via links, so may be relevant.
The brake transaxle shift interlock (BTSI) solenoid is energized by the body control module (BCM) switching circuit 816 to ground. When energized, the BTSI solenoid will allow shifting from PARK. A condition for shifting from PARK is that the foot brake must be applied. The BCM receives this information over the serial data link from the engine controller.
The BCM also monitors ignition 1 voltage, circuit 339 and accessory, circuit 743 to determine if the ignition switch is in the RUN or ACC position.