A vehicle control system (VCS) in a passenger compartment interfaces with underhood components using a Bluetooth® link. The underhood components include a siren assembly with a sound generator and backup battery. The battery enables the siren assembly to function after the vehicle's battery is disabled. The siren assembly includes a Bluetooth® transceiver for establishing the link with the VCS. The siren assembly monitors the hood of the vehicle through a hood sensor, to detect unauthorized opening. In response to such opening, the siren assembly transmits an alarm to the VCS, which transmits an alarm to a user's mobile device and/or a security service. The sound generator may also be activated, by itself or under control of the VCS. The siren assembly connects to the engine computer/ignition module, to allow the VCS to monitor engine speed, and facilitate proper start of the engine when the VCS receives a remote start command.