MetroMPG
Found that Merc thread with the tail light ducting below,
http://ecomodder.com/forum/showthrea...trol-6075.html
The tiny slits they have would not be capable of any serious ducting, I think thaey were there primarily to give clear seperation at the body edge, like vortex generators, so rather than putting a sharp edge on the lights like they are doing moreso now, they chose to eject some sharp straeams of air, which simulate a sharp edge and give clear seperation of air from the body.
That's my interpretation.
My direction would require quite a bit more volume, to help drop pressure at rear and and hence reduce drag, more like the uk study linked in one of the posts above..