Looking at the pattern of dirt on the rear screen of a Saab 9-3/900 I owned, it was noticeable that it wasn't nearly as dirty as the rest of the car and there was a faint pattern to the dirt.
Attaching wool-tufts to the screen with insulating tape (and finding a willing driver) showed that whilst air in near the centre-line of the car did follow the curve of the car at low speeds it became detached at higher speeds (the tufts hardly moving). On either side of the rear screen there was a vortex where air came in from the side of the car into the low pressure aft of the screen. Something like this:
Having paid an additional £9.63 to the insurance company before doing so (how did they work that out?) an array of vortex generators were sprayed body-coloured and fixed to the rear of the roof - where I thought things might start detaching. I angled the generators so that they presented an angle of attack that would generate vortices to 'unwind' the vortices from the side of the car. The rear window even got wet when it rained and some more wool tufts showed most of the flow to be in the right direction.