Nail polish remover (acetone... sp?) with some cotton swabs will work to take off the yellow dullness. Just be very generous with the cotton swabs (just one swipe). You can leave fine scratches if you swipe with the same swab more then 2-3 times if there is a lot of yellow film.
I did this to my mom's Altima and it took the film off instantly. The lights look like new. I just washed them off and applied a coat of wax and buffed to finish them up.