@kk_83
Thanks that is my friends car, you can clean the inside of the headlights with water (which is common over here), but i havn't done it. For the outside you can use some plastic polish for it. Best speak to some honda civic (96-01) owners and see what they have to say since their headlights are also plastic and tend to discolour.
From what i've heard on the mercedes forums (e-klasse) they use some mild abbrasive waxes like a mixture of brasso liquid and car wax/polish.