Genuine Mercedes special Oring seal for the bowden cable housing where it connects to the right side of the automatic transmission. This is a common leak area and you may be fooled into thinking it is leaking out of the pan gasket.
Replacement is not easy without the right tool and instructions. It can be a little "tricky" getting the bowden cable out and back in just to replace this inexpensive o-ring.
For Kent's video instructions as well as the special tool you should have on hand to complete this job see related products.