Body Control Module (BCM)
WARNING: This page is about a different car, the 2007 GMC Sierra and 2007 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
The CTD system is an internal function of the body control module (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. Following the 30-second alarm period is a 10-second time-out. The system will then re-arm.