GM Service Manual Online
For 1990-2009 cars only

The rear hatch ajar indicator will illuminate when the rear hatch is open and the ignition is in the RUN position.

When the rear hatch is opened and the ignition switch is in the RUN position the liftgate door lock ajar switch closes and provides ground for the ajar switch signal in the instrument cluster. The instrument cluster will illuminate the rear hatch ajar indicator.