I have an older Benz ('85 380SE), and had an even older one ('76 450SE). Both have/had sunroofs. If yours does, sometimes the drain tube(s) can get blocked, causing water to accumulate in the rocker panels and seep under the floor carpeting.

Another possible is the front or rear window seals. If parked on an incline, depending on whether the front or rear is up, the water will 'flow' front to back or back to front. Very frustrating to trouble shoot.

On my old '76, there was a rust hole somewhere on the firewall, behind the fuse box, that always caused water to seep in and soak the front. I gave up finding that one.

Try taking the carpeting up front and back, with a dry floor, and soak the car with a hose. Maybe you'll get lucky and find the source.

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1986 560SL
2002 Toyota Camry
1993 Lexus