[Christ]

I'm going to go all out and suggest boat tailing the wheel fairings in the rear bumper and rocker panels. They're already tailed up on both parts, but (especially in the rear) more could be done.

I'm also going to note that because of the rear design of the car, if anything were to help rear flow off the tail, it would likely be a spoiler-like device mounted at the base of the windshield, probably no more than 1" from the deck lid to promote linearity through the course of the roof to decklid transition. A rigid decklid extension would clean up the detachment point considerably, if you're seeing dirt devils in the tail lights.

Under the rear bumper, the curve transitions to a fairly sharp rounded edge... usually, a square edge would be of benefit, although increasing wake area, it will separate flow more cleanly, inducing less of a vacuum... A rear valance splitter-type spoiler might be effective at that aft curve.

Those are about the only rear-end suggestions I have, other than whale-tail tapering.

EDIT: After further consideration, I can almost guarantee your tail lights are getting dirt devils at their tops, and it's because of the low-speed flow profile at the bottom of the rear bumper. If you straighten out that aft edge, your license plate and lights should stay almost 100% cleaner.