[aerohead]

Once the air gets to the point of maximum greenhouse cross-section it is 'leeward' of the body,no longer attacking, and it can't 'push' the water as it would in the forebody.
If you were already in motion when the rain began,the water would be managed by the drip rail system (hidden in modern cars since Hucho's work) or blown over the captured-vortex and the captured-vortex atop the trunklid would help to centrifuge the rainwater away,with only moving air licking the surface of the backlight.
If you started out in a rain,this vorticity would eventually dry the backlight if your velocity were sufficient enough to outrun the momentum of the water.
If you drive a square-back, the wake turbulence will transport water pumped by the tires up from the road surface to totally obscure your view.Something which the re-attached flow on the 300's boot trailing edge prevents.