No experience with your particular year/model, but most leaks come from a bad rear window seal, either the rubber or rust-out in the lip/channel at the lower edge or corners, or, and this is far more likely, the rear light housing gaskets, where the plastic assembly joins the body.
If it's the light housing, just unbolt the whole thing, clean it all up, and use silicone sealant around the edges where the gasket is (or was). Unlikely, but it could be a torn or worn out trunk seal rubber, easily checked visually.
Hard to do in winter, but someone inside the trunk with another spraying water all over outside might reveal the leak, if it's coming from the window glass seal or the rubber lid seal.

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1986 560SL
2002 Toyota Camry
1993 Lexus