Door seals probably. Have you checked the trunk for any wetness at the same time? If it's wet there as well, your third brake light is leaking from the mounting studs.
Yeah I did have a leak in my trunk from the break light and the license plate grommet. My driveway is a steep incline and have it parked with the trunk on the downhill so water shouldn’t be running from the trunk. I’ll have to check the door seals again.Door seals probably. Have you checked the trunk for any wetness at the same time? If it's wet there as well, your third brake light is leaking from the mounting studs.
MIne was the drivers side upper door w/sI just got my 09 ctsv 2 weeks ago and it is kept outside, so when it rains my rear floorboards are soaked. I’ve tried to simulate rain at a car wash as well as wetting the car with the hose but can’t seem to track where it’s coming from. The car doesn’t have the pano roof.