An auxiliary device for opening and closing the loading compartment of a motor vehicle, which can be installed on the back door of the vehicle, comprises: an anchoring plate (4) installed on the fixed side (PF) of the back door, and an anchoring plate (3) installed on the mobile side (PM) of the back door; a case (1) that protects and completes the entire device, said case (1) being hinged to the anchoring plate (3) of the mobile side (PM) of the back door; an engagement and release system constituted by a fixed plate (10) and a mobile plate (9), which is connected to a cylinder (6) rotatable with a key (2), and a thrust spring (7) set on the case (1) of the mobile side (PM) of the back door, and by a mobile shaped hook (5), provided with spring-operated return means, which is mounted on the anchoring plate (4) of the fixed side (PF) of the back door; as well as a removable armoured hinge (8), with a system for protection against non-authorized removal, made up of an appropriately shaped and structured pin with breakable head (13) and a groove for an anti-removal safety Allen screw (14); a chip (11) fixed within the case (1) hinged on the anchoring plate (3) of the mobile side (PM) of the back door; and a feeler (12) appropriately shaped with elastic electric contacts fixed on the anchoring plate (4) of the fixed side (PF) of the back door and electrically connected to a processor (CPU) set on the fixed side (PF) of the back door or in the loading compartment.