Courtesy/Reading Lamps
WARNING: This page is about a different car, the 2007 Pontiac Montana SV6, 2007 Chevrolet Uplander, and 2007 Buick Terraza. However, it is still accessible from the selected car via links, so may be relevant.
The courtesy/reading lamps receive voltage in 2 ways. The reading lamps receive voltage on the inadvertent power courtesy lamp circuit, which has voltage any time the BCM is awake, and are turned ON by the individual switch on the lamp assembly. The courtesy lamps receive voltage on the courtesy lamp high control circuit, controlled by the BCM.