Things you will need to do it cheap:
-Fine wet/dry paper around 1500 grade.
-Cut and polish and or Polish
-Wet your light and have a hose that is trickling water onto where you are rubbing your light with the wet/dry paper. Rub in small circles around the whole light. You will see and feel the difference as you go along. When you are done, move to the next light.
-Finish the job by drying the lights and then polish with the "cut and polish" or with the polish and you are good to go.
Some polishes have a UV protector put into them but if not, you will find after a couple of weeks they will start to dull up again. If they do, simply polish them and it will come up clearer.
A mate of mine went berserk on his lights and stuffed them due to using a rougher grade of paper and some chemical. Just be careful that what you use does not attack plastic!
Final note, plastic / polycarbonate lenses have a UV protector layer (as mentioned above) and this has deteriorated which causes your lights to go dull / yellow. Also, plastic deteriorates over time being in the weather. You can get polishes with a UV protector it it but if your after a long term type fix then best off to find a proper lens sealer. Companies that work with plastic will be able to get you a proper sealant or worst case, use the stuff that is in the kits from Supercheap / Autobarn etc.