No need to make anything fancy for the back of the car. Just tape some yarn to the back window, just below the spoiler, the bumper etc. The more tufts, the better ( but not too much ! )
If you feel like it though, there has been some discussion about the angle of the Prius rear glass not being the 'optimal' angle, so it would be interesting if you added a sort of 'spine' down the center of the car that has tufts. Im on vacation in Canada ( eh ! ) at the moment and can't draw you a good image of what i mean, but imagine a piece of cardboard spine that stands up several inches on the rear glass looking something sort of like this :
You could 'sew' the yarn through the cardboard and this would keep the yarn from twisting around at any odd angles that result from the way the yarn is taped to the car.
BTW, This is an example of overkill that most likely creates drag from the tufts themselves skewing the results :
If you feel like seeing the airflow behing the rear of the car farther back than the surface of the car, you can have the 'spine' attach to a flat section of cardboard and sew tufts in it as well. ( Basically a sheet of cardboard attached to the back of the car and oriented vertically. )