The lenses on the headlights of my 2002 PT Cruiser turned "milky". One Chrysler dealer quoted replacement headlight modules at about $250 each. Another Chrysler dealer suggested a $15 kit of fine grit sandpaper and a buffing compound to remove scratches.
It helped, but just for a while. If I rub a lubricant such as WD-40 on the lenses, they get clearer, but just for a while. I consider the reduced lighting a safety hazard, and have considered using more powerful bulbs. I am willing to buy new headlight modules, but am concerned that the problem will repeat itself. Have you any suggestions?
Late model cars and trucks use clear plastic (polycarbonate) headlamp lenses. This material is much more durable against impact from road debris (rocks from the truck ahead of you) than glass, but they do tend to yellow, scratch and fog up from the elements as time goes on. They're also sensitive to harsh chemical cleaners.
Replacement polycarbonate lenses are expensive, with no guarantee that the new one is going to stay clear any longer than the original. But there are a bunch of aftermarket products claiming to restore your lenses to near-new condition. Some begin by sanding the exterior surface and following up with finer polishes (LensRenew - www.lensrenew.com - $32.95), and others are more simple with a single liquid polish (Meguiar's PlastX - www.meguiars.com - $5.49). Meguiar's also sells a Mirror Glaze cleaner and polish, each $7.99, which they consider to be a more professional two-stage process.
Never attempt to install a more powerful bulb than the lens was originally designed for. The additional heat may cause damage. Also, be sure there's not a fog from moisture on the inside of the lens. This can be a normal condition if it's a slight fog that clears up with headlamp operation. If it doesn't, there may be a leak in the lens housing that will require replacement to fix. And the WD-40 is only producing a temporary wet gloss which may inflict further damage in the long run.
As far as spending the money for new lenses, I'd try another one of the aftermarket products first -- preferably a different type and brand then the first attempt. It may be worth the effort, even if it means rubbing out your lenses on an annual basis.