you can clean the outside with toothpaste. it will help. cleaned my van right up. cleaned the outside of my gtp and made a noticeable difference. however, if the inside is fogged or hazed up you can take your light assembly out and bake them in the oven for like 10 minutes on low heat (like 200) to separate the lense from the frame. clean them up, and glue them back together and then seal them with silicone or something.
i'm pretty sure there is a write up in the how to on how to bake them.