Cargo Door Description and Operation
WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2003 GMC Yukon XL, 2003 GMC Yukon, 2003 Chevrolet Tahoe, 2003 Chevrolet Suburban, and 2003 Chevrolet Avalanche. However, it is still accessible from the selected car via links, so may be relevant.
The Body Control Module (BCM) monitors the lift glass and the cargo door ajar switches. When the switch closes, the path to ground is completed, and the BCM sends a class 2 message to the instrument cluster to display the CARGO DOOR AJAR message.