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If they're not under clearcoat it's an easy job with a simple blowtorch and some brake cleaner from the auto store.
Use the torch to evenly heat the metal behind and around the sticker, then simply peel it off by picking at a corner with your nails. Make sure you don't start melting/burning the decal with the torch. You have to be brief with it, just to soften the sticker up and heat the metal.
Use the brake cleaner and a cloth/some sort of plastic edge that won't scratch the paint (like a ruler for example) to scrub the glue left behind. The brake cleaner makes decal glue turn into a jelly, so it will come off relatively easy after a few sprays and scrubs.
If the decals are really old and cooked on by the sun you're going to have a very tough time getting them off and will probably end up burning them on. And in that case I'd actually recommend not messing around with them.