Trunk lid fins idea.
 

Maybe Ive been up to long and thinking to hard... but I know about the rear diffuser underneath the cars can help, but what about some vertical fins on top of the trunk lid? I got the idea because I just have bolts covering the holes where my old stock spoiler was, I could use them to support fins. Does anybody think it could do any good? I dunno if splitting up the air on that part of the car would help? The fins might make a vortex (like wiglets do on airplanes) ,I dont see how that could hurt, yet I haven't done to much research or testing. I just want to get some feedback on the idea.

fins
 

The drag which can be streamlined originates behind the car and is a function of the base pressure of the wake.
To lower the drag,the separation point on the cars body must be moved aft,while simultaneously reducing the body cross-section in such a way as to prevent separation,yielding a smaller wake of higher pressure.
Fins can't do this.
They could affect flow onto the rear deck area as with the C-pillar fins on Ferrari's 599XX racer.
Trouble is the cost of the wind tunnel time to dial them in.Which could cost $1,000,000.

Alright, thanks for the feedback!

Wind tunnel time may cost big buck$, but tuft and coast down testing can be had for cheap. So, why not work up some cheapo fins out of Coro, duct tape them to the car, and see what happens?