I had the same question. Turns out it's an 8mm nut on the upper inside of the rear fender. It is likely hidden beneath a few layers of undercoating and muck. I don't know if you can get to it at all without removing the rear wheel. These photos show what you're dealing with:
Click to view attachment Click to view attachment Don't forget that at the top (the phillips screw) there's a plastic barrel spacer between the trim and the body. It's there to prevent over-tightening the screw and bending the trim. Don't loose it when you finally remove the trim. Or if you loose it, don't forget to make/buy another one before you reinstall the trim.