Abstract: A rear door assembly for a truck cap for mounting onto a truck bed is provided with a specially modified plate disposed at a lower edge of the rear door of the truck cap for cooperatively engaging protective ribs that are disposed on upper ends of tailgates of late model trucks. The enclosure plates disposed on the rear doors of the truck caps extend outwardly and downwardly so as to conform to the general shapes of said currently-available tailgate protector ribs. The enclosure plates enable the rear door to close fully and provide the requisite engagement between the lower end of the rear door and the upper end of the tailgate so as to prevent moisture and dirt from entering the inside of the truck cap.