When you remove the interior door panels to reveal the door cavity, check your window 'frame'. The lower metal strip that is glued to the window pane bottom and attaches to the regulator arm.
If that is rusted, it may indicate that your window rubber moldings are allowing water inside the door cavity when it rains.
Ergo it also accumulates at the bottom.

If bad, try regluing to glass because at some point the glass may slip out of it. (happened to me. I replaced glass with junque yard parts), also installed new moldings.

If you don't have small drain hole at the bottom, just drill out one or two and treat the edges with the POR so it does not rust.

Good luck.

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1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
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1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD