I've just picked up a Lexus IS250 as a courtesy car whilst my dads RX is in for a safety recall on the brakes. Although not an estate they have a camera between the lights, but still recessed above the number plate. It comes on during reverse and there are two grids on the screen. The one is fixed and looks to be the size of the car, and the other adjusts with the steering wheel to give the direction the car will travel.
I can take a picture of the mounting if its any use. I know its not a tourer, but you may be able to fabricate something similar.
Some of the options on the car are really amazing. The voice command is great. Shame the car is quite plastic inside really.