I’m sure someone else has thought of this before, but, just in case …
When I had a set of old yellowed decals, I tried placing the sheet in a window to bleach them out. However, even in Hawaii, the sunlight at the window did not do the trick. Then it occurred to me that my car sits in the sun all day while I am at work. So, I taped the decals to the inside of my windshield. Within a couple of days, the yellowing was significantly less.
Tip submitted by:Richard MorimotoMililani, Hawaii