In the process of replacing/upgrading the radio in my 1991 300d, I pulled all of the trunk liners. Now, with a full view of the fuel tank, I can tell that the sending unit seal is weeping somewhat. There's a ring of seeping diesel that has spread out around the sending unit opening... slightly wet to the touch and smells like diesel.

I've been searching here (and on other parts sites) and can't seem to locate a sending unit seal. I assume it's just and o ring of some sort. Can someone point me in the right direction?

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1991 300D 2.5 "Rocinante"
2002 Golf TDI "Teen Spirit"
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1984 300D
1966 Mustang I6 3sp
1985 Mazda RX-7 GSLSE
1982 Toyota Supra
1977 Datsun 280z
1971 Datsun 240z