Dirty marks under headlight covers, how 2 fix

[Veilside R33]

hey, bought a skyline on friday and need to clean up the headlights, they have dirty marks underneath the front plastic casing/cover, looks like a few years worth of the weather getting in, the covers dont unclip or unscrew, i believe they r glued shut anyway, so my question is....

How do i get the cover off to polish up/remove the grime from my covers, its a beautifully maintained car so this shits me as the headlights look the way they do especially with the lights on as it makes it even more obvious,

so do i have to have them removed by a pro or is there a way?

[(B1) MR_fanny]

this has been covered before....do a search. some say to use some sort of polish like plastic polish or cut'n'polish on em. my drivers side is shitted and theres no way of cleanin them apart from crackin it open with a heatgun or somesort. im just waitin till a second hand one is available.