[cdru]

Depending on the age, you may be able to remove the front lens cover. On older vans, they fall off on their own due to brittle glue. It may take a little prying, or I've also heard gently warming it in a oven (very low temps) also will loosen the glue. Then you should be able to use a plastic polish available at autopart stores. It's made just for that, cleaning and polishing plastic lens assemblies.