when a decal has been on glass for a while and is exposed to
the weather, it gets etched, from sand, gravel, dirt, elements
in the wind, and it will leave the impression of the decal,
its there for good. 8)

I guess I have to go tell my wife I need new glass in the '68 now. :D

You could try a buffer with fine water base rubbing compound and finish up with cotton wheel and fine jewelers rouge. Look at it this way, they are junk now what harm can you do.