Same problem on my 92 300D. Like you, I had water leak that took me a while to find....even replaced windshield at indie's suggestion. And like you, I'm cheap, but even worse: I used a ghetto combo of POR-15 and duct tape! I'll have to "upgrade" to the glued plate and undercoating....

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 160k miles
06 E320 CDI "Rutherford", Black on Tan, 180k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 79k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife