I vote for a rear window sticker too. The local university issues a parking sticker that is sticky backed and goes on the outside of the window. I have to change parking stickers each semister. They are relatively easy to remove with a razor blade knife. A sticker that goes on the inside usually runs into the rear window defogger elements and is almost impossible to remove without damaging the rear window defogger. (Previous owner's kid stuck window stickers over the defogger elements and I could not get them off completely.)