Apparatus and method for mounting a plastic panel upon an underlying metal substructure includes fixing the longitudinal location of one end of the plastic panel by machining a locating feature in the metal substructure at a precise location relative the adjacent body panel and then engaging an integrally molded locating element of the plastic panel with the machined locating feature. A screw is then installed between the plastic panel and the substructure. A mounting member for the rear portion of the plastic panel includes a first molded plastic block slidable on a support bracket of the substructure and having an integrally molded spring finger engaging the bracket to resiliently urge the mounting block to a certain longitudinal position on the metal substructure and a second mounting block which is connected thereto by a screw thread by which a rotation of the second mounting block establishes a mounting face thereon at a precise position transversely of the vehicle body for abutment by and support of the plastic panel in precise surface alignment with the adjacent body panel. The mounting face of the second mounting member provides a field for the drilling of a hole for receiving a fastener attaching the plastic panel to the mounting member. Accordingly, thermal or hygroscopic growth of the plastic panel induces longitudinal movement of the mounting member so that the plastic panel is movably supported for distortion-free growth relative the underlying metal substructure.