I made a rough 'wind tunnel' with some plasticine, a cutting board, and running water. I just printed out a malibu side profile, cut it out, set it on 1/4" thick plasticine, and carved it out. Running the water was a little tricky. I tried to keep it far enough up stream and straight ahead of the model.
I got a baseline, in which flow detaches from the roof. It's several inches ahead of the rear windshield, which is what I expected.
I followed up with a 'vortex generator' in clean 'air'. I was elated to see that the flow then followed the back window and stayed attached until the trailing edge of the trunk lid
Of course I have to do tuft testing to properly locate airtabs (or some such junk) and this is very promising. Ideally I would take my car to a full sized wind tunnel to confirm.