[Dragrad]

Thanks to Reebs....

The trick is to pour water over the front facing surface of the sticker, then offer it up to the inside of the rear window glass while still wet.

Position until happy, then peel off the backing paper while the sticker is attached to the window, and finally use a credit card or similar to smooth out any air bubbles or excess water trapped between the sticker and the glass.

The finished result should look smooth and the sticker background White.