My uncle was a used car dealer for 30 years, and here is the trick that he has shown me.
Rub toothpaste on one headlight for 3-5 minutes with a soft rag (NO whitening, baking soda/peroxide, ect). Use either tartar protection or regular.
Once you have completed the rubbing, wash it off with baby shampoo and water. Hand dry.
They will look new.