Have the plastic lenses gone opaque? I have seen a few posts from A6 C5 owners and Passat owners complaining of this. They report success using polishing compounds on the surface of the headlamp, followed by a sealing agent to try to prevent re-occurrence. I can't remember this being reported on any C6 models, but then maybe the climate over there promotes the effect more?

A couple of posts are here:

headlight poor
Frosted glass ?

Just a thought,

MF.