Application of Vortex Generators to a blunt body
Here's a paper that should help you to understand VG's better. i.e., they don't work. Also note page 8, in their workup, they talk about the "Rear Edge Radius" and it's effects. It is plain to see that the bigger it is, the better. So looking again at the Holden, you can see it has a
Very small radius.
The Renault is bigger.
But your Echo takes the cake.
This is why I was saying, doesn't much matter what you do, Toyota has optimized the design pretty well. It is certainly no accident that the rear edge radius on you roof is comparatively massive. I figure it close to 1 meter.