MBV8

Hi I have a Mercedes 2013 with a 3.0cdi engine ( om642). It’s leaking a lot of oil but I’m a little unsure if it really is the oil cooler sealings. There is a little oil around the drain hole on the left side of the engine but there is a lot of oil inside the bellhouse. If the oil cooler is leaking I guess it would be oil outside the engine and bellhousing and not inside the bellhouse. When I remove the black rubber cap located down on the bellhouse there is a lot of oil inside the bellhouse. I’m thinking it should be the rear crankshaft sealing leaking or can there be so much oil inside the bellhouse when oil cooler sealings are leaking?