Filling in the wake?
 

My daily driver is a '92 Chrysler LeBaron sedan (see pic in the garage).
Aerodynamically, it has one glaring flaw that I have been thinking about for some time - the rear glass is near vertical. Judging by the actions of dew droplets on my rear glass, the airflow over my rear glass and trunklid is entirely seperated anywhere above about 45-50mph.

What I have been thinking of is to create some sort of vent that would connect the underbody of the car to the trunklid area just behind the rear glass. In theory, I believe that this would reduce the low pressure area behind the window providing both a reduction in drag and in lift. I plan to take some basic pressure measurements using a water manometer to verify a pressure gradient between these two places before cutting holes, but I was wondering what other peoples ideas are as well. Any thoughts?

some would say VGs to create vortexes in that area. I am still unsure of these as I have no experience with it.

another thought is to make a plexi glass fairing (I hope I said that right) that extends from the top of the cab as far back as you can stand. not sure if you use your trunk but if you don't, you could go all the way back to there angling it down slightly (I have heard 10 degrees but that may be wrong). others have done this in cars like the metro to get better FE. and basjoos has taken it to an entire boat tail design which has put him in the top 10 (at least he used to be, not sure now) his car is "keeps on rollin'". I don't recomend doing that but you could use the plexiglass to make a slight extension even if you do have to use the trunk.

https://www.gassavers.org/garage/view/272 this is not my car but hopefully it will give you an idea of what I am talking about. also plexi-glass is clear so it may be safer.

I don't think you're going to be able to use large enough ducts and vents to do the job in a useful way. A fairing would probably be better but of course it's going to be ugly.

Late 60's Olds Toronado had just such louvers, but for interior "flow-thru" venting, not undercar air diversion.