A fastener for joining trim pieces to a motor vehicle includes a metal insert molded into an elastomeric body of a fastener. An anti-rotational rib prevents excessive rotation of the fastener after installation. Oppositely extending flexible wings of the metal insert are spaced from each other to allow full deflection upon insert of the fastener without interference between the flexible wings. Protective guards formed at distal ends of a pair of support legs provide improved insertion alignment for the fastener as well as protection between the fastener and metal surfaces of the vehicle. In another embodiment of the present invention, a dog house assembly is provided having aperture and slots mating with the installation features of the sill plate fastener. In still another embodiment, a method for installing a sill plate molding into a vehicle using a fastener of the present invention is provided.