Got to admit I've never had a saloon car with a Rear Wiper and I've owned quite a few. I would of thought it's quite difficult to mount a wiper motor on a car with a boot as the rear screen angle is steeper on a saloon than a hatchback. Also, in a hatchback the rear hatch has hydraulic props which keep the heavy hatch lid from closing on your head. A large majority of saloons don't have this so the extra weight of a wiper motor assembly would almost certainly make it impossible to stay open. Boots are rather flimsy as well not like a hatch door which is framed and skinned.
Just whack on the rear screen demister, I find mines that good that unless it's torrential it usually sorts out the rear screen well enough even in the rain.
AB