If they're badly yellowed, use 1000 then 3000 grit wet/dry sandpaper to clean them up. Then finish with buffing compound. It made a big difference on my pickup. Then I switched to LED lights and really got some light out of them compared to the stock lights.

“In a time of deceit telling the truth is a revolutionary act.”
― George Orwell

It's not over when you lose. It's over when you quit.