@Chaka
I only restored the external surface. The headlamps are completely sealed so the inner surface is not discoloured.
@Njunge
Brasso is simply a mild solvent and an extremely fine abrasive.
The misty appearance on faded headlamps (read faded plastic) is caused by fine scratches on the surface. The brown-ish discolouration is due to oxidation, including fine dust getting trapped inside those scratches.
Brasso evens out those fine scratches such that fine dust cannot hold onto the plastic headlamp. It also disolves the thin oxidised layer that forms due to exposure to sunlight (UV), rain etc
Just apply & rub the stuff using cotton wool, leave it for a 1-2 minutes to dry into a haze, then rub it off / buff with a soft dry cloth. If in doubt, try it out on a small inconspicuous corner of the headlamp.
This is not guess work, its something I'm still doing. Read about "Other Uses of Brasso" on
http://en.wikipedia.org/wiki/Brasso