The control unit is activated if there is a ground signal from any of the 4 doors (door jamb switches), trunk (trunk lamp switch), hood (seperate switch for anti-theft alarm), and possibly it may be activated by the ignition switch if the alarm is activate, and possibly by the brake pedal switch. There may also be a way the alarm knows the radio has been removed. No glass breakage sensor or anti-tow sensor on one that old, at least on the US versions.

Gilly