My first expert has got back to me (Professor Joseph Katz).
Maybe my first email wasn't clear because in a second email I added an image and also asked:
Is there any relationship between the surface body pressures we can measure at A (top of car near rear) and B (side of car near rear) and C (base pressure)? For example if we increased pressures at A and B, would we see an increase at C?
Professor Katz said:
The pressures at C should be close to B because the separation bubble is influenced by the outer flow.
The pressure at point A depends more on the rear geometry - is there a vortex there? what is the spoiler angle, etc.