The reason sedan don't have rear wipers is because of aerodynamics.
The rear windows, which is like mostly vertical, doesn't get that much rain, because you're driving FORWARD. Very little water actually land on rear window. There's also rain-run-off channels on the roof of the car to "guide" the water away from the top of the roof, then out to the sides. The roof is actually curved, like the road. So most water don't even runoff to the rear windows.
And another factor is the trunk itself, which helps move air along and not circling around the rear window, trapping dirt and rain. Air flows past the roof and flies for a bit then drops onto the trunk. There's a bit of a "vacuum" near the rear window, but not enough.
Hatchbacks, with sharply raked windows, have much bigger surface area so rain can drop onto the rear window itself. There is also no 'trunk' to guide the airflow. All of the air flows right at the surface of the window, so it does need rear wipers.
SUVs/wagons 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. Wiper's not for the rain, but rather for the dirt (so it also has washer nozzles)
Kasey C, PC guru since Apple II days
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