This is something that might be overlooked if you're trying to reduce weight in your vehicle but if you have pretty much any 4 door car this is a good way to get rid of at least 4lbs or more.
You've probably noticed that the back windows of most cars won't roll down fully in some cars this is because of the shape or angle of the door not allowing the glass to fit fully inside the door but in other cars this is because of a stopper block that keeps the windows from making it all the way down. Supposedly this is a safety feature so if you follow the instructions is this and someone falls out your window because of it I am not responsible.
Anyway what you want to do first is a little research on your car to find out if you have a removable stopper or not. If you do keep reading, if you don't know if you do keep reading, if you don't have a stopper sorry this won't help you any.
Next you need to either find out how to remove it online and know what to do or find out how to remove it by taking stuff out of your car until the back window disappears into the door (hint, you'll find what you're looking for in the back door).
Start by removing the door cards every car is a little different but usually there are a few bolts or screws and a bunch or plastic fasteners holding it on. Once that's off you'll have to pull the vapor barrier out of the way I just pull it gently till the adhesive lets go then you can just reseal it when you finish. Now there should be two bolts holding the stopper in so just take them out and then pull the stopper out. At this point you'll want to CAREFULLY roll the window down to make sure it works and won't scratch the glass or anything. If you have no issues with the test roll the window up and down a few more times just to be safe then put everything back together and carefully test it again. If you have no issues then you're done.
Here is a 2 part video that shows how to do this on a cavalier or sunfire you could use it as a reference for the steps in this post if you want.
Part 1
Part 2