GM Service Manual Online
For 1990-2009 cars only

When the ignition switch is in the RUN position and the rear hatch is open the ajar switch for the hatch closes. When the hatch ajar switch is closed ground is provided through G401, circuit 450, the ajar switch, and circuit 9804 to the Instrument Cluster (IPC). When the IPC receives the ground input from circuit 9804 it turns on the rear hatch ajar message.