did this a couple of months ago on an ex gf's jeep. there is a UV film on the headlights that yellows over many years. get some sand paper in grits of 320, 600, 1200, 2000 and a sanding block from autozone. tape off the area around the headlights, then get a garden hose and just barely get a trickle of water going on the headlights and wet sand with the sandpaper on the sanding block in a circular motion and go through the sand papers from lager number to smaller. at 2000 the headlight will have very vine scratches on it, then use 3m rubbing compound (or any other type of liquid scratch remover) and a rag and rub it in until the scratches dissapear and you are left with shiney headlights.