Stop water intrusion!
Check third brake light, spoiler, seals, plugs around the door. If down door frame (pillar), check seal and plug around door frame and roof rack bolts. I once saw where a body shop, didn't replace a plug along top of door frame. A lot of water would run in, a whole lot.
If the water is filling entire 1/4 tip, even if not. Remove 1/4 tail lights. Clean out quart tips, draining, by remove the 30mm rubber plug at bottom.
The 1/4 tips also have weep holes in the pinch weld. Over time 1/4 tips fill with dust in bottoms. These weep points (plug & pinch weld), clog with dust. Which then, the water is retained within the dust/mud in 1/4 tips. Resulting in rusting out of 1/4 tip. So even if water not filling entire 1/4 tip. It's a very good idea to clean out 1/4 tip. Also worth spraying (if no rust) with 3M body cavity wax or one of the many undercarriage oils (Wool Wax, fluid film, Blaster, etc.). If rusted, it's better to mitigate rust first. Which will likely involve some sort of painting.
PS usually not to badly rusted
Keep in mind. The RH rear quarter tip, is in area of exhaust tail pipe. Exhaust can enter cabin from opening in rear area, which includes hatch seals, 1/4 tip and wing windows. OM warns us: "Do not have rear wing windows open, with engine running". Reason; Exhaust fumes can enter cabin. We see most of the leading edge moldings on these side wing windows, cracked! Why: Because side wings are being, left open while driving. Which they're hinges are not designed for.
The PS side, we see the most moisture and rust in. Due to Rear AC unit. But, I've found the worst side. Is if some opening into underside of 1/4 tip. These opens have been holes drill for wiring and not sealed, hits the rear bumper, Hits, damage quart tips, opening their seams. Any opening, allows in more dust and water in. The dust builds and retains the moisture.
Keeping 1/4's clean inside and out and protecting, is our best defense. Against rusted out quart tips.
Open to cabin, which is next to tail pipe.