A lock mechanism for a motor vehicle ignition switch includes a housing defining a barrel and a locking cylinder fitted for rotation in the barrel. A cap is fitted over one end of the housing capturing a frangible lock insert between the housing and cap. The frangible lock insert includes an electrically conductive pathway between radially displaced connection points on the rim of the insert. A circuit includes the electrically conductive pathway which is interrupted by disintegration of the frangible lock insert. Such disintegration occurs upon efforts to tamper with the lock mechanism or application of brute force to the sensor.