I used Meguiars Plastix with very good results on mine:
http://www.meguiars.com/en/automotiv...leaner-polish/
You only need a small amount
soft micro fiber cloth
don't press too hard, let the polish do the work
each time you use it you will get a little more clarity in the plastic so use once or twice to gauge your results. Horn buttons with more wear will require a few applications.
Mine came out like brand new after spending a little time with it.
The plastic tabbed piece is not being recreated so you have to restore what you have or find an original replacement. They get brittle with age so be gentle.