If you are replacing it because it is cloudy, what you need to do is clean it real good inside and out. Don't use anything abrasive, but you do need to rub alot. I also rain-x the outside when I am done. If you are replacing it because it is damaged, I mean ripped, because if it is just scratched the scratches will fade with repeated cleaning. I've been told that replacing is a pita. And the savings may not be worth it if it leaks and ruins the interior.
Personally the rear window is the most cleaned part of my car. I even keep a squeedge in the trunk to clean it. I also use the squeedge to get ice and snow off the rear window as well.