Body Control Module
WARNING: This page is about a different car, the 2001 Cadillac Eldorado. However, it is still accessible from the selected car via links, so may be relevant.
Content Theft Deterrent (CTD) system is an internal function of Body Control Module (BCM). When an unauthorized entry is detected, BCM applies ground to horn relay, park light relay and headlight relay. BCM applies ground once per second for 3 minutes, or until a valid input to disarm system is received.