Summary:
No matter what the reason for putting that bumper sticker on your car, it seems like after such a period of time, the message you have stuck on your car seems to have faded already - maybe both literally and figuratively. If that happens, taking the faded sticker will be a good idea.
You may have put in that bumper sticker on your vehicle for various reasons. You may have simply liked the message that the sticker told the world. Another reason could be that you just would not like to hurt your son's feelings when he presented you with that "I Love My Dad" bumper sticker on Fathers' Day. Or simply because you just were part of that organization whose campaign was to "Save The Whales".
No matter what the reason for putting that bumper sticker on your car, it seems like after such a period of time, the message you have stuck on your car seems to have faded already - maybe both literally and figuratively. You may not be supporting such a campaign at present anymore. Or maybe, people may not be able to decipher just what your bumper sticker said. Or what it actually tried to say in the first place. It has simply worn off that it just looks like a big piece of paper plastered on your car and you just want to take that off.
You can take off that faded bumper sticker by scraping it off the glass. To do this, you may use a razor blade. But make sure that you use a razor blade only if the bumper sticker has been placed on a window. However, if the sticker has been placed on a bumper, you can try to peel it off first with your fingers. Remember, there are rubber bumper stickers and this kind is easy to peel.
However, if you try to peel them off and it does not come off, you can use water. Have a basin filled with boiling hot water. You should protect your hands with some rubber gloves while you do this. Also have a dip rag or a sponge which you would be wetting on the water. Then, press this against the bumper sticker for around ten minutes or so. After that, you can use a plastic spatula or putty knife to peel away the sticker. Make sure that you do this slowly to avoid scratching or chipping your car's paint job.