I picked an red LED 3rd brake light up off ebay, wired it in parallel with
the right rear tail light.  The nice thing about the LED's is that they are
so low power you don't have to worry about splicing off the existing wiring.
I trimmed out the window tinting where the light sits.

The biggest problem I've had is figuring out how to keep it glued to the
glass.  Suggestions for gluing plastic (mounting brackets) to glass?

I don't think I'd want it "on" the rear shelf for two reasons.  1) It'll be
dimmer than if mounted flush to glass especially if you don't trim out the
window tinting and trimming out the window tinting without mounting it flush
is going to look funny.  2) If it's not mounted flush to the glass you'll
get some reflection off the window and into your rear view mirror.  Since
mine is presently "not" glued to the window I might try it on the rear deck
just to see how it is, but I'm not optimistic about it.

IMHO, external mounting would be ideal, but then you have to worry about
weatherproofness and getting a wire to it.

-Jon