A decorative trim strip assembly for vehicles is provided, having a laterally curved portion formed therein. The assembly comprises a metallic backing plate and a polymeric resinous strip element (usually polyvinyl chloride) bonded to it. At least one longitudinal slit is formed in the polymeric resinous material so as to form longitudinal strip portions therein, in the area of the curved portion, and each of the longitudinal strip portions is stressed to an extent less than that which would cause undesirable deformation. In forming the decorative strip, the slit polymeric material is placed into a jig, with the ends of the slit portions thereof extending to varying amounts beyond the curve, the jigged element is then bonded to the backing plate (usually by heat bonding), and any excess amount of polymeric material extending beyond the curved portion which is not required is then trimmed off.