Circuit/System Description
WARNING: This page is about a different car, the 2008 GMC Sierra, 2008 GMC Cab & Chassis Sierra, 2008 Chevrolet Silverado, and 2008 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
Voltage for the courtesy lamps is supplied from body control module (BCM). When a door is opened, a signal is applied to the BCM. The BCM then applies voltage to the courtesy lamps voltage supply circuit and the components on that circuit.