Abstract: An anti-theft device for an automobile audio system automatically prevents the audio system from functioning whenever the audio system is removed from its authorized vehicle and placed into another vehicle. This anti-theft device includes a microprocessor which operates with a bus interface for communicating a vehicle identifier from a vehicle multiplex bus to the central microprocessor. The device also has non-volatile memory within which the first received or authorized vehicle identifier is stored. The subsequently received identifiers are stored on RAM. A battery detector connected to the microprocessor alerts the microprocessor whenever the audio system has been disconnected from the battery source.