[ecomodded]

Thanks for your input Kach, and bringing that to my attention. I see your point, I also noticed one more flaw with my add on, the sides of the Beetle actual do have a defined trailing edge.. and my extension would only deny the airflow its already good flow path off the side of the car. Where it does meet with some roof edge flow and as you stated and the cars wake in the rain confirms that a large vortex spirals off the cars top two corners.

My new thought is to make a smaller extension that will attempt to keep the roof edge airflow in line with the roofs airflow. A small change in the right direction I now deduce..
A video of the car cruising on the hwy in heavy rain would determine the/any changes.