I can only tell you my experience with my Supra which may or may not apply. My Supra had the EXACT same symptoms you describe. The problem in my case was the gaskets around the tail light assemblies. They had decayed and were leaking. I could see some faint telltail "stain" where the water was leaking in. I took it all apart, cleaned a dried everything, RTV'd the gaskets to the assembly and then put silicone grease on the side that mates to the car. Tightened the bolts and no more leak!!

MP

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra