IDK how they did it, but I had a paint shop remove some of mine that were custom (non factory) and paint them on with chromolusion (color changing paint) ..... I would not recomend this to anyone wanting to save a dime, the paint shop did not realize how much labor was involved in the chromolusion paint, but they stuck to the original quote. Said it should of cost dubble
I knew a man years ago said he used egg

to remove the peeling decals from his Bounder
I did tho find this for you, I hope it helps you out
How to Use Vinyl Off to Remove Sign and Vehicle Graphics: 7 steps