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.

__________________
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