I'm looking at my analysis of the car.
The book,'THE NEW VW BEETLE', by Jonathan Wood, page 35, has a fine color photograph of your car inside VW's climatic wind tunnel, with smoke flow.
The afterbody reaches a tangent angle as steep as 44.5-degrees from the horizontal.
The flow is separating at a tangent angle of 24-degrees, almost-exactly half-way down the backlight.
Without the wing, you're at Cd 0.39.
If you follow Dr. Hermann Burst 'solution' for the equally nasty 1972, Porsche 911 Carrera RS 2.7, @ Cd 0.40, you'd follow the local streamline, as far as you were willing, filling all the void in between with body, to create a Kammback.
Carl Breer did this for the Chrysler Airflow and was able to reduce drag, from Cd 0.51, to Cd 0.244 ( essentially a 2012 Tesla Model S, but in 1934 )
All PGA golf balls experience flow separation at 23.5-degrees.
W.A. Mair recommended no more than a 22-degree afterbody tangent angle.
I'm conservative, and happy to limit my projects to 22-degrees.