Anne: I cut & paste this from this site. Scroll down and maybe you will see it from Barry. Here it is: Much sloshing sounds of water in the Cascada at the weekend. In the rear near side (offside for the Buick’s) quarter, in front of the wheel. I had a look underneath and there was a bung on the sill. It came out with a wiggle and dumped about five litres of clean water on the road. No more sloshing sounds, other side is dry.
It leaves the odd dribble from the drains behind the wheel arch covers but this is the first time the panel has filled up. With the door open it looks like there is a gap in the seal. It’s a 2013 Car so no warranty or recall possible, but I’ve bought a cover. Might also try and adjust the door so it closes a bit tighter.
However, what sounded like a big problem, was very easy to solve, so if you hear water in the rear quarters... there are drains! Good luck anne. bob y