Had a look at the headlights on my car. As per the clio my wife had, the lenses have started to slightly dull.
Cure - Solvol-Autosol metal polish. This is a light abrasive and cleans them right up.( If heavier dulling just add a green scrubber used to wash the dishes.) Once done, polish with ordinary car polish.
PS, if on a budget, try a scubber with toothpolish. Seriously. lol