Door Latch (Door Jamb) Switches
WARNING: This page is about a different car, the 2003 GMC Sierra, 2003 GMC Cab & Chassis Sierra, 2003 Chevrolet Silverado, and 2003 Chevrolet Cab & Chassis Silverado. 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.