CHECK OUT WHY HATCHBACKS HAVE REAR WINDOW WIPER BUT SEDANS DON'T !!!

Rear wipers are essentially meant for vehicles with an upright rear window that collects dust, dirt, snow, etc., because of aerodynamics – the flow of air over the right angle where the roof and rear window meet creates a low pressure area that pulls that nasty stuff in and deposits it onto the surface. 

Sedans, with their sloped roofline rely on that same flow of air – this time over the roof and down the window to keep it clean. Adding a wiper would create more aerodynamic drag creating slightly more fuel use. A wiper also requires an electric motor and a fairly convoluted mechanism and lengthy arm to make the wiper sweep across the glass from side to side. 

SUVs/hatchbacks with window right at the rear has nearly vertical windows, but not the trunk to guide the airflow. It has a low-pressure zone right in that area when it's moving at speed, which will "suck in" water and dirt and thus make the window very dirty.

This is totally based on the concept of aerodynamics !!