Diagnostic Aids
WARNING: This page is about a different car, the 2010 GMC Sierra, 2010 GMC Cab & Chassis Sierra, 2010 Chevrolet Silverado, and 2010 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
Inspect the trailer brake output supply voltage control circuit (dark blue wire circuit 47) that runs from the trailer brake relay to the 7-pin trailer connector in the bumper for water intrusion or corrosion.