If it's dripping as far back into the trunk as the wheel wells, it's the rear windshield seal. If it's further towards you, it's possible it's just the seal around the trunk lid. If you look carefully under the lip where the trunk lid seal is, you'll notice that the metal is made of multiple pieces, and so not necessarily watertight.

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1982 240D automatic
Odometer quit at 307K...about 325K now