Abstract: To retain an accessory apparatus including a tape unit, such as a tape deck forming part of an automobile radio, in a vehicle, such as a car or a boat, a dummy cassette (17) is provided, having an externally visible lock (18) which operates a locking bolt (23) which, upon lock operation, deflects from an aligned position with the dummy cassette through a window in the accessory or tape deck housing and through a holding frame (11) or holding strap (30) secured in the vehicle. The windows (27, 27', 27") in the dummy cassette, the accessory apparatus including the tape deck and the frame or strap in the vehicle are in alignment so that the bolt can be passed therethrough, thus preventing removal of the accessory apparatus from the vehicle. Preferably, the lock is so placed that it is visible, so that it will be immediately apparent that the accessory apparatus is locked against removal.