Fuel Sending Unit Seal?
 

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?

You're correct that it's an O-ring. You might try just giving the sending unit a good snug and see if it stops. The one on my '86 was still flexible and supple, so I just put it back in when I cleaned my sender.

The seal is in a weird place on Pelican's site, it's under "Electrical" since it's tied to the sender. From my '86, but your '91 should use the same seal: https://www.pelicanparts.com/catalog/SuperCat/3342/MBZ_3342_ELSENS_pg1.htm#item3

That's the ticket... many thanks! I'll try snugging it up first.