I've had good results with any kind of citrus based sticker/adhesive removers in spray cans, not harsh on the paint underneath, and dissolves the sticker glue in a while, you will need to apply several or even many doses and let it soak between, until the edges starts to come loose, and then put on more, repeat, repeat. Can't of course be sure it will work on such an old sticker, but with lots of patience I think it will work since no sticker that I have wanted removed is still in place.
However, I would be careful with getting the citrus on the edges of the sound deadening, would not be fun if that started coming loose.