Circuit/System Verification
WARNING: This page is about a different car, the 2009 GMC Sierra, 2009 GMC Cab & Chassis Sierra, 2009 Chevrolet Silverado, and 2009 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
Ignition ON, command the Rear Red LED, Rear Yellow LED 1, and Rear Yellow LED 2 ON and OFF with a scan tool. The appropriate indicator should turn on and off as commanded.