If the lens is cloudy on the inside, there is an outgassing problem with the materials used in the light source. Youll probably have to replace the lens.
On the outside, there are many products out there. I used a 3M product on my passat headlights and the stuff worked increadible. Its basically 3 steps of fine sandpaper (up to 3000 grit) finished off with a buffing compound. My lenses went from not being able to see the bulb to almost crystal clear. The only issue is that by doing this, you are removing the clear coat that is providing UV protection for the lens.