Our new Hundai develped puddles on the back seat floor after the first carwash in early December. We did not find the water until the next day. We took it to the dealer and waited until they had a loaner. They held the car and claimed to have run it thru their car wash several times and run a hose over it with no water. We took the car back and went immediately to our regular car wash. No water right after the wash, but the next morning puddles again on back seat floor. While waiting to again get loaner and return the car, we tried to replicate problem and no water showed up. Two days age, we found a lot of puddling under the rubber floor mat on the driver side floor. We went back to the dealer, who again put car thru their car wash and came up with the suggestion that we stop going to the car wash that we have gone to for over 5 years, never having a problem with our 9 year old car which certainly should have had loose gaskets or some old age problem. Needless to say, we consider the dealers solution totally bogus and are pursuing our complaint. What could this problem be?
One possibly useful trick is to close all windows and run the car with max AC. Then pour soapy water on every roof and trunk seam. The bubbling can be furious and funny if it isn’t your car. The water can come in through the tail light assembly and leak into the trunk too. Then you stop the car and the water flows right in past the back seat.
Does it have a sun roof. It sounds like a blocked sunroof drain to me. There are four of them, one at each corner. They are usually cleared using string trimmer string or compressed air. Either method and cause trouble if you are not careful.