Citroen XM Forum

Car failed the MOT not able to test light's alinement, the plastic inserts have gone yellow, two are needed a left and right.

Try removing them and dipping in bromide to remove the yellowing.
Also polishing can remove this, though obviously that is difficult on the patterned side.

I'm not sure whether the yellowing is due to the plastic itself discolouring, or whether there a UV coating applied, which breaks down over time.

You might find the headlamp output has been significantly reduced due to dusty internal reflectors. Wash them out by swilling with soapy, then clean water. Don't rub the silvering, it will come off.

I will give that try, bromide where can I get that from and how do you use it

Apologies, I told you wrong information, there.
Bromide is the compound leaching out of the plastic, causing the yellowing. What you need to do is bleach that with Hydrogen Peroxide, using sunlight as the catalyst to activate it.
Completely submerge the plastic in a transparent tub of Hydrogen Peroxide, then leave in the sunshine. The stronger the Hydrogen Peroxide, the stronger the sunshine and the warmer the solution, the quicker the bleaching effect.

Still a chemical that will need to be required