Body Control Module (BCM)
WARNING: This page is about a different car, the 2009 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.
The CTD system is an internal function of the BCM which utilizes serial data and various switch inputs to perform CTD functions. When the BCM detects an unauthorized entry, it activates the horns and exterior lamps for 30 seconds. The system will re-arm immediately following an alarm event.