I'm unclear why you're dead-set on taking air from the base of the rear window (which, unlike the base of the front windshield, does not see high pressure), and what you mean by "reducing a low pressure area indirectly." To pull air from that location, you will need to create even lower pressure downstream, and you've selected a spot that is guaranteed to have lower pressure than almost anywhere else on the body, especially given the Dart's backlight shape (concave). If you manage to do that, that will almost certainly increase drag, since you'll be exacerbating the pressure differential between the front and rear of the car and possibly disrupting pressure recovery further back on the trunklid. You wrote that "aero DF" trumps everything, but it doesn't--if you're searching for faster lap times (which, if that's not the case, why are you tracking?), balance is everything. If a duct cools the rear brakes but increases drag and you end up with a slower lap time because of it, what good is that?
So everyone can get a visual, here's the back of a 1972 Dart:
I think you would be much better off taking in air in the extensive real estate available in front of the rear wheel arch (less drag, higher inlet pressure, shorter duct with less internal drag, no need for fans and further drag/power consumption). But, I would also back up and wait until you actually
have a problem with rear brake temperatures to try and address.
As far as clean airflow for a wing--have you experimented with a wing? Is there a height limit? Have you looked at solutions like Mitsubishi's vortex generators at the top of the backlight, which are intended to address just such a situation as this (backlight too steep)?
If you can get your hands on a Magnahelic and take pressure readings along the centerline of the body while you drive, that might be illuminating.