Why is water accumulating on the floor behind the driver seat? on 2003 Mercedes-Benz CL500

At first I thought a valet left the rear window open, I brought the vehicle for repairs, the carpets were removed and dried and all the electrical was replaced (apparently the majority of it runs underneath the back seat). Yesterday I noticed water had accumulated again.

No water in front pass side? Sun roof? Only after rain or car wash?
No, only the back on the floor. However the car was washed at the dealership the day before. Is it possible water is not draining properly and accumulating only in that area?
1 more answer , 3 more comments
poss heater core is leaking and running back there under the carpet. are you adding water to the cooling system?
poss window or door seal leaking?
No, there's no moisture anywhere except the floor.
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