On a Sprinter.
Air Tabs will have a more pronounced effect on a vehicle where the side walls and the rear meet at right angles to the airflow, like with a box on a straight truck or a trailer. And you should mount them as close to the back (trailing) edge of the vehicle while ensuring proper adhesion. On a Sprinter, the rear-most side walls of the box begin curving towards the back end, instead of meeting the back at right angles. But since you need an absolutely flat surface with which to mount the Air Tabs, you cannot mount them on the curve (the Air Tabs are not moldable and will not stay stuck if forced onto a curved surface). Because of that curve on a Sprinter, you cannot mount the Air Tabs as far rearward as is recommended. You have to move them forward a few inches and mount them there.
The Air Tabs must be mounted within 14 inches of the rear right angle, but the closer the better. An in or two is the ideal. With a Sprinter, the rear isn't even a right angle, so 14 inches is too far. The only viable option is to mount the Air Tabs immediately in front of where the curvature of the rear side walls begin. Moving them forward decreases their impact on the airflow drag a little. Not a lot, but just a little. They are still quite effective when mounted as shown in the photos below. The bottom picture also shows the roof-mounted tabs.