I have fixed mine. Gone through 3rd brake light, privacy screen, hatch etc but still there. Mine ended up being the metal piece between the front and rear fixed window that rattles below/behind where the seat belt comes out and sounds plasticy but it actually isnt.

Try this fix. When driving, hold onto the metal vertical upright in between the windows really hard, I pushed mine out against the windows and rattle disappears. If this works then unclip top cover on seatbelt and tighted the bolt holding the bottom of this metal piece in place....no more rattle. Did it on both left and right sides and both bolts were loose enough to cause rattle over undulating surfaces.

Easy to do, just follow your nose to bottom of that metal piece and tighten bolt

Good Luck