Circuit/System Verification
WARNING: This page is about a different car, the 2010 Saturn Outlook, 2010 GMC Acadia, 2010 Chevrolet Traverse, and 2010 Buick Enclave. However, it is still accessible from the selected car via links, so may be relevant.
Observe the scan tool Dimming Control Signal parameter. The reading should change as the dimming is increased and decreased.