I suspect your car is similar to what some call an
econobox.
Samples:
Car Wind Tunnel Data
Surf around the forum to find typical solutions to similar vehicles.
What ever you do either needs to be in the airflow, stepped back slightly from the surface plane, or at the surface plane (
like the roof).
Doing something way at the back in a area out of the flow isn't going to do anything but rob you of time and money.