well if ur talking bout the plastic beein yellowish coz its old

then its pretty easy, i used some chrome polish for them
couple of minutes and they are as new

the top layer of plastic becomes yellowish as its old, so if u remove it the lenses will be sweet
im not sure what exactly i used, but it was some polish thingy with brass in it or something

im sure if u ask ur local parts shop they mite help u

hope it helps