Gears.
The best thing to do with plastic screens and covers is keep them clean by washing them with lots of water with screen wash in to prevent scratching, but you need to wash the body afterwards to stop the screen wash leaving stains. You can also get a cleaner/coating for plastic which stops UV from damaging it.
If your plastic is badly scratched you can get cleaners to remove the scratches and I have heard that you can use tooth paste to renovate headlights but the thought of doing the rear screen with a tooth brush is not funny, although the thought of the look on neighbour's faces if they see you cleaning your car with a tooth brush is.
Mick