A mounting frame member is provided for framing and sealing a frameless window pane of an after-market motor vehicle tailgate intended to replace a tailgate having a different structure. The mounting frame member includes a mounting frame body mounted to the motor vehicle, the mounting frame body including an integrally formed sealing weatherstrip at an inner peripheral region thereof. A supplemental sealing weatherstrip may be provided adjacent the sealing weatherstrip of the mounting frame body and over the frameless window pane. Accordingly, a single, unitary BIW structure may be utilized when effectuating after-market structural changes or replacements to an existing tailgate.