^
Good suggestions!
I usually try to assess the TYPE of decal first (on paint). Believe it or not, a pic would help.
Glass is easy - a razor blade, followed by brake cleaner, GooGone, or any other solvent - followed by a good glass cleaner. I like Invisible Glass or SprayWay for cleaning glass, and newspaper is WAY better than microfiber towels, paper towels, or anything else.