Is the water in both wheel wells or just on the right side? If just the right it could be a disconnected hose from the fuel overflow drain. Water gets behind the fuel door when it rains and instead of draining out the bottom of the car it ends up in the trunk.

I had this problem in my W116. I assume there is the same drain in the W123 but I don't recall for sure.

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Joe

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1980 300SD - 495k miles - 'The Ambassador'
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Former Family Members
95 C280
73 280SEL
90 300D
87 300SDL (X2)
86 560SEL
84 300D
80 300SD

There is a small hole, 1/8", under each side of the rear window for draining the seal channel. Look up unto the trunk with the liner removed and you will see them. When my seal was leaking I could feel moisture at those holes. Also the rear seal has a depression at the bottom sides of the window where water can pool up and filter down between the glass and the seal. I sealed my windows with permatex ultra black by masking off the body and the chrome and applying a thick bead with gloved finger. Time intensive but the fix was low cost, and if done neatly and trimmed, will go unnoticed.