Most often, I'v seen them mounted on the metal part of the rear mudflap. However, the ridges that run from top to bottom, while seemingly a good place for the bolt holes, are about 1/4 inch too narrow. What I did to solve that is to get some flat strap at Tractor Supply and drill the coresponding plate holes in exactly the same place width-wise on the flat stock. Then as close to that in the mettal portion of the mudflap, thus the bolts didn't twist sideways and funk up the plate. The drill was the same size as the bolts, so as not to be able to twist sideway, thus the bar stock forced the bolts to tighten in a straight path saving the plate from being dis-figured or twisted.