The rear main seal allows the oil pump to supply oil to the rear main crankshaft bearing without shooting the oil out the rear of the engine. It seals between the radius in the block for the crank on top, the radius in the oil pan on the bottom and to the inside the rotating surface of the crankshaft.

You may be able to take off an inspection plate or a place provided for draining the torque converter to look up into that space to see if you can determine where the oil is coming from.

But first you should attempt to catch some of the fluid dripping from beneath your car on a clean piece of paper... you may be able to tell by the color and texture whether it is trans fluid or engine oil. Trans fluid typically has a red tint to it... and Diesel oil is typically filthy black looking.

I don't know about your model for sure.. but in the past when this was discussed concerning MB diesels I believe it was determined that the rear seal is designed with a locating pin in the upper radius which 'locates' the seal . So the old fashioned capability on some of engines of 'rolling' out the old seal and 'rolling' in a new one from underneath is not an option. The crank must actually be lowered some from block in order to put the upper half of the seal in ...and it may even be a round seal instead of upper and lower halves.

If it had been the rollable type then you could have taken just the oil pan off to replace it. If it had just been the round type you could have just taken the transmission off to reach it from the rear. But with the crank needing to be lowered I suspect that taking out the engine is the most dependable way to address it. I am sure someone will speak up if they have managed to address it while still in the car. But some people on here can set the valves with just straight wrenches and would not dream of owning the spring retainer tool... so some of these things are relative to a person's stamina and other things....of course , if I was going to lower the crank I would certainly look at putting new crank and rod bearings in just as a matter of philosophy.... Greg