As was noted above they were originally stapled below the 1/4 panel tailight to the flange on the body behind the bumpers.
The staple holes should still be visible on the body flange.
I have never installed on my restorations/repainting the cars since I do not plan on driving my cars at speed on gravel roads. They intended I think to prevent rocks, gravel from hitting the painted body (on Belair and Biscaynes ) and the SS or Impala trim pieces around the taillights from being hit.
Paul