I have a 1993 corolla I took the headlights apart and painted the chrome part black and they look good, but the headlight cover has some yellow grime and somebody told me to WET sand it with 2000 grid sand paper and it would come off is that true?

Wet sanding will give the covers a frosted appearance, and I do not think that would look good.

Are the covers removable? Replacing them may be a better option.

Otherwise you could experiment with different cleaners but because the covers are plastic you need to be careful not to use anything that can damage the plastic.

Take rubbing compound and spend alittle time polishing them up is what I did and they shine nice and new.