Here's a quite good photo (not mine) of how the rear end looks:
The fact that Saab also build airplanes is clearly seen in the design of the cars of the older days. The basic concept of model 96 was created in the late 1940's. The body shape is smooth and looks aerodynamic but in reality I think the rear part is a disaster. What could have been a perfect shape has been distorted short, or concentrated, in order to fit a practical passenger car.
Attaching a boattail/kammback would probably improve aerodynamics a lot. I have decided to do this in small but less rewarding steps to make it easier to the rest of the family but also to avoid starting up another big project I can't finnish... Starting with the rear window as an isolated module seems therefor right.