Courtesy Lamps Manual Operation
WARNING: This page is about a different car, the 2002 Oldsmobile Intrigue. However, it is still accessible from the selected car via links, so may be relevant.
The courtesy lamps may be manually operated by the dimmer switch. When the dimmer switch is placed in the ON position the body control module (BCM) receives a ground from the courtesy lamp switch signal circuit. Upon receiving this signal the BCM then pulls low the interior lamp control circuit, which illuminates the courtesy lamps. The voltage for all interior lamps is supplied by the courtesy lamp supply voltage circuit of the BCM, whenever the BCM is active. The dimmer switch will override any BCM operation of the interior lamps, such as entry illumination.