I know this is late to the party, but I actually ran across your post while researching something else.
If you haven't solved your problem yet, this may be an answer and has worked for me. It is a long thread, but I have a youtube video (post #34) posted in it, where I have it installed in the rear window of my TT, for the reasons you speak of. Other members also have pics posted of the same in that thread.
Made a wireless backup camera system cheaply
Hope it helps