Door Latch (Door Jamb) Switches
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 theft deterrent system uses the door key switch as one method in order to activate the alarm. If the BCM detects any door opening before the BCM receives a disarm input, the alarm mode activates.