The neighbors have one of these parked in back. Example:
http://www.usedracingcar.com/detail/spoilers-wings/01-02-SATURN-S-SERIES-COUPE-REAR-SPOILER-CPE-SC2_281836103357.html
It strikes me as somewhat strange; The part on the deck lid is weathered like a bumper, not a fender. The shape of the fender requires the bolt-on part for continuity.
It appears that it was designed to protect pedestrians if you back into them, but only in the middle.
OTOH the top is a perfect teardrop shape with a dead-flat beltline. I'd love to graft one into the top of a school bus.