I replaced the rear taillights two days ago and I had trouble finding a writeup with a picture, so I figure ill post this up in case anyone ever needs a reference in the future. First the picture, then the writeup:
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1) Remove wheels and passenger side rear wheel well shield.
2) Undo wingnuts on rear sidemarkers from inside wheel well and hang lights outside of quarter panel.
3) BlueCircles: Phillips head screws removed from top where bumper cover meets hatch.
4) Red Circles: 8mm nuts removed from inside the wheel wells. (Soak in liquid wrench first)
5) Green Circles: 13mm (I think) nuts removed from underneath and behind bumper cover.
6) Yellow Circles: 19mm (I think) nuts LOOSENED from underneath bumperettes. (They don't need to be completely removed)
7) Bumper cover is now free. Pull out far enough to reach in and unclip harness for license plate lights.
Taillights are attached to bumper cover with 4 nuts, 2 on top 2 on bottom. i would suggest removing these when the bumper is completely off. Unclip the light harnesses from inside the hatch, behind the covers on each side and push harness through the frame.
9) With all harnesses unclipped and nothing going through the frame, the bumper cover with taillights attached is completely free.
Those bumper covers are freaking tough to get off there!! I don't know which is worse; the front or the rear. Thank god Porsche decided to use plastic bumpers on the 928. Had they been metal, I believe they would've tack welded the bumpers to the 1/4 panels!