There are lots of threads on this topic.

Basically, if the water is in the wells, check the tail-light gaskets. If it is in the center of the trunk, like mine, it is the rear window seal. There are exceptions.

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1983 300SD "Guderian"
1987 MR2
2015 Camry
2015 Chevy Spark
2006 Hyundai Tucson