How to fix Ford C Max , Focus and others poor number plate lights output.

Whilst pre-checking the car for the upcoming MOT I noticed very little light output from the number plate lights.
On closer inspection the inside of the light lens was covered in a brown film and the bulbs had slightly blackened.
Took them indoors and cleaned the lens with some detergent and a tooth brush in the sink, came out much better. Fitted two new bulbs and that will do, you can get LED fitments if you wanted off ebay.