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, characterized in that it comprises, in combination: - an anchoring plate installed on the fixed side of the back door, and an anchoring plate installed on the mobile side of the back door; - a case that protects and completes the entire device, said case being hinged to the anchoring plate of the mobile side of the back door; - an engagement and release system, constituted by: - a fixed plate and a mobile plate, which is connected to a cylinder that can be turned with a key, and a thrust spring set on the case on the mobile side of the back door; and - a shaped mobile hook, provided with spring-operated return means, which is mounted on the anchoring plate on the fixed side of the back door; - a removable armoured hinge, with system for protection against non-authorized removal, made up of a pin appropriately shaped and structured with breakable head and groove for anti-removal safety Allen screw; - a chip, which is fixed within the case that is hinged on the anchoring plate on the mobile side of the back door; and - a feeler, which is appropriately shaped with elastic electrical contacts that are fixed on the anchoring plate on the fixed side of the back door and is electrically connected to a processor set on the fixed side of the back door or in the loading compartment; wherein said device has a mobile part hinged on the mobile side of the back door and not fixed thereon, enabling the case to turn; and wherein the spring-operated engagement/release system is mounted on the fixed side of the back door, and is hence fixed with respect thereto; wherein: - when the device is in the closing position, the electrical contact between the chip and the processor is closed via the feeler and the signal sent is that of "device closed"; - when the device is released in the opening position, and can thus turn about the hinge, the electrical contact between the chip and the processor is open, and the signal sent is that of "device open". - an anchoring plate installed on the fixed side of the back door, and an anchoring plate installed on the mobile side of the back door; - a case that protects and completes the entire device, said case being hinged to the anchoring plate of the mobile side of the back door; - an engagement and release system, constituted by: - a fixed plate and a mobile plate, which is connected to a cylinder that can be turned with a key, and a thrust spring set on the case on the mobile side of the back door; and - a shaped mobile hook, provided with spring-operated return means, which is mounted on the anchoring plate on the fixed side of the back door; - a fixed plate and a mobile plate, which is connected to a cylinder that can be turned with a key, and a thrust spring set on the case on the mobile side of the back door; and - a shaped mobile hook, provided with spring-operated return means, which is mounted on the anchoring plate on the fixed side of the back door; - a removable armoured hinge, with system for protection against non-authorized removal, made up of a pin appropriately shaped and structured with breakable head and groove for anti-removal safety Allen screw; - a chip, which is fixed within the case that is hinged on the anchoring plate on the mobile side of the back door; and - a feeler, which is appropriately shaped with elastic electrical contacts that are fixed on the anchoring plate on the fixed side of the back door and is electrically connected to a processor set on the fixed side of the back door or in the loading compartment; wherein said device has a mobile part hinged on the mobile side of the back door and not fixed thereon, enabling the case to turn; and wherein the spring-operated engagement/release system is mounted on the fixed side of the back door, and is hence fixed with respect thereto; wherein: - when the device is in the closing position, the electrical contact between the chip and the processor is closed via the feeler and the signal sent is that of "device closed"; - when the device is released in the opening position, and can thus turn about the hinge, the electrical contact between the chip and the processor is open, and the signal sent is that of "device open". - when the device is in the closing position, the electrical contact between the chip and the processor is closed via the feeler and the signal sent is that of "device closed"; - when the device is released in the opening position, and can thus turn about the hinge, the electrical contact between the chip and the processor is open, and the signal sent is that of "device open".