Abstract: A motor vehicle includes a folding passenger seat assembly having a seat portion and a backrest portion. The backrest portion has a left end and a right end both of which are hinged to the seat portion so that the backrest portion can be folded from a vertical use position to a horizontal non-use position. A child safety seat is secured to the backrest portion. The left hinged end of the backrest portion is provided with a manual latch for selectively locking the left end of the backrest portion in the vertical use position. The right end of the backrest cushion is provided with an inertial latch for automatically locking the right end of the backrest portion in its vertical use position only in response to sudden deceleration of the motor vehicle. Weight, component cost and labor are conserved by installing the manual latch on only the left end of the backrest portion to lock it in its vertical use position.