My best guess is that detachment of air occurs about middle of the rear window.
If this is the case, a mid window foil or spoiler would work best.
If you use this proposed spoiler at the top of the window, you may be missing out and increasing the drag.
Try to find yourself a similar coupe in wind tunnel images thread so you can see what I mean.
http://ecomodder.com/forum/showthrea...ead-26678.html