addSome 'angles' at the back of cars can produce what Hucho calls 'bi-stable flow',where the separation line will dance around.
A 'flap' can trigger positive separation (Chrysler calls it 'burst'),fixing a constant coefficient of drag.
I suspect that during development work within the new aero-acoustic wind tunnels,that they were picking up cyclic 'booming' behind the original stylists design,and the flap is a cheap fix for bad aerodynamic design.
Kamm/Fachsenfeld studied these flaps in the late 1930s.
What's old is new again!
Here is the 1930s 'flap'
