Completely dry the car out. Then take a few gallon jugs of water and start pouring them on the car at various places and see what happens. Vents below windshield, sunroof, doors, hood hinge drains, etc.

I've been trying to find my leak. rear driver's side foot well was always flooding. It turns out that the drain under the drivers' side hood hinge was clogged. The drain runs under the plastic wheel well liner. It was clogged and the standing water rusted a hole thru the fire wall. It was a pretty good size hole right next to the parking brake pedal mount. Anyway, rain was draining into this rust hole and making its way back to the rear floor. Strange as the drivers floor never got wet. Water somehow got from the firewall to the rear.

Used lots of that 2 part "steel" putty stuff from Autozone to fill the hole and seal the fire wall. And cleaned out the drain.

That was yesterday's project. I will see how it holds up.

__________________
1987 300TDT
1981 VW MKI Caddy 1.6 diesel, waiting on engine swap
1983 D-50 Power Ram 4x4 "Mitsubishi" 2.3 turbo diesel
assorted gas powered crap and motorcycles

RIP: 1984 300TDT, 1982 300TDT, 1984 190D 2.2, 1992 300D 2.5, 1987 300TDT, 1982 Maxima LD28, 1983 Maxima LD28, Isuzu C223 P'ups X3, 1983 Holiday Rambler 6.2 Banks turbo diesel, 1984 Winnebago LeSharo 2.1 TD, 1985 Allegro 6.5