Most likely one of the drainage tubes is blocked.
Remove wipers and the cover.
Clean the space below.
Use a flexible rod and a lot of water to unblock the drain tube (left an right corners).

Rob

Another possibility to look for while you have that plastic cowl cover off is behind the drain hole up under the windshield. I had a hole rusted through on my passenger side, drains were clear but R/R floor was getting soaked after a rain or wash job. Rust hole was fixed and interior has been bone dry since

This 300 SDL just goes and goes.... I replaced the shocks with Koni's several years ago and that was the best thing that i've done to the car .Great ride and great handling. I add 1/2 quart of syn oil at oil changes(dino oil) and that quiets the engine. Get a Johnson Controls battery under the brand name of Interstate, Die Hard, & several other brands and it will last over eight years per my experience.

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Paul 1987 300 SDL; 2000 ML; '69 MGB; '68 VW Fastback

Purolator primary filters are clear,and catch more stuff,than german filters,you can clean them with gasoline.Hide your egr,disabling,by using a BB in the vac hose.Egr's are slow death for diesels.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

Use a transparent pre filter so that you can see into the filter .I now have over 55,000 on my pre fuel filter and I see very little junk in the filter.But, the main fuel filter doesn't last 25,000. However, I use quality (Shell) fuel only unless I'm on a trip. The price of fuel is about the same locally.

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Paul 1987 300 SDL; 2000 ML; '69 MGB; '68 VW Fastback