The plastic covers on my zenon headlights are starting to haze over has anyone else experienced this and does anyone know how to clean them up or can you renew just the outer covering of the headlamps without renewing the whole unit

 

Meguiars and 3M both do headlamp restoration kits just for this sort of thing

 

Start with 400 grade sand paper, wet.

Move on to 800 and then finally 1200. Use plenty of water.

Then finish off with some compound polish, g3 or similar.

Come up as good as new