Here's how I do it.
1st.
Spray the lense with a heavy duty cleaner. I use Purple Power. You see a lot of the yellowing, flow off the lense. Rinse well after the cleaner has time to work.
2nd.
Now using 800 grit wet sand paper, sand the lense smooth.
3rd.
Using 1000 grit wet sand paper further smooth out the surface of the plastic lense.
4th
Rinse and dry the surface of the lense.
Now use a product called Meguriar's PlastX Clear Plastic Cleaner and Polish. Apply with a soft cloth and polish and buff the surface of the lense. Continue application of polish and you will be amazed at what will happen.
Your clear plastc lense will become clear and shiney again. Looks great and you'll restore a lot of your night vision due to the lense being clear again.
Hope this helps. I was taught this by a friend who does this at car lots and gets $50.00 a car. If you like the results you don't have to pay me, a mere thank you will suffice
Ron