This isn't what you'd call traditional design, but the car's LAMM (look at my motor) window is now in place. The window curves a tiny bit front to back so water can drain off the glass. The drains also act as vents (theoretically) to help fill in airflow across the center portion of the rear deck (air moving over the rear deck's surface in S1 and S2 cars is back to front). The counter intuitive movement sets up an eddy that supports air flowing over the roof, reducing turbulence and low pressure drag behind the rear window. That's also the reason for the lip at the rear. It's not a spoiler in the classic sense. When the car's on the road, I'll do some wind testing to see if my ideas are valid.
