Buy a polishing kit and go at it, if it's the clear cover that has yellowed. About $10-20 depending on the brand and you can get them at Home Depot or you local auto parts store. I bought the Turtle Wax brand.
It's not hard to do and a LOT cheaper than new units. I recently did a 99 Nissan Altima, an 01 Mitsu Galant, and my 2000 F-250 in about an hour. Read the directions and take your time. I taped up the paint around the light so I didn't polish it when I began to rub. You wash the dirt off the light, polish it clear, then use the clear sealer to finish it. Use a bucket or something to sit on and do it in the shade so you give it enough time.
The kit uses a very fine grit polish that is wet like you might use for jewelry. Mine worked out pretty good. Just take your time.
My wife's Toyota Sienna is next.