I did three of them when I did my rebuild, and all three were a disaster getting off. Assuming they are the originals, any adhesive underneath will be dried up and solidified. They are backed with a very pliable metal; almost foil like. For the effort and potential damage to your paint, I'd just order new ones. At $12-15 each, it's worth it. Still add a little heat when you take them off though... Not too much, but just enough.
I know it's not the answer you're looking for, but it'll be the way to go.