I had the same problem when I bought mine about 2 years ago, actually accumulated enough water to corrode some wires in the harness which what was supposedly my starting issue. I took it back to the dealership and after about 2 months of going in and out they slowed the leak down, something with the taillights seal but it still leaks. I have tried to find out where its coming from but can't. Was the car ever in an accident? Mine came up clear on Carfax but there is a new quarter panel so I know it was hit pretty hard. I think I have read something about water can leak in from the rear surround, maybe check there also. Good luck and don't let too much build up in there.