A lock system for a hinged window-mounted back door is disclosed which comprises a first lock mounted to a main door part for releasably holding the main door part in its closed position, a second lock mounted to the main door part for releasably holding an auxiliary door part in its closed position, a control device mounted to the main door part for releasing the latched condition of the first lock when assuming a first position and releasing the latched condition of said second lock when assuming a second position, and a restraining structure associated with one of the first and second locks for preventing the control device from assuming a position to allow one of the first and second locks to assume its unlatched condition when the other lock assumes its unlatched condition.