Yes, the rears most likely drain down the "C" pillar. That is where my 240 w/sunroof drained. Open the trunk and look for the tubes behind the rear seat near each wheel.

As Spook mentioned, it could also be the sunroof or door seals. A wet headliner in the sunroof area probably means a bad seal or the rear drains are clogged and backing up. Do note that my 240 needed new sunroof seals and it only leaked on the seat when I washed my car. Little to no water on the floor.

I suspect in your case it could be a door seal especially since you said it only leaks on one side, correct? One last thought. Have you removed or damaged the door sleeve (inner plastic liner inside the door panel)? If it isn't tightly sealed then water can leak out of the bottom of the door panel onto the floor. Check the bottom of the door panel for dampness/water damage to confirm this. Hey, hey ask me how I know this....:)