Denial is a powerful force. Your leak is caused by a rusted out channel around your rear window. All the sealer in the world applied from the outside will be ineffective. Had identical on my 300CD.

It is fixable (I did) but not inexpensive. Rear window must be removed. I had a windshield pro do it to prevent breakage since the coupe rear windshield is rare and expensive. A good body shop can remove the rust and rebuild the channel. Add a new seal and it is done.

Cost:
R&R rear window approx $100.
Repair rust approx $350 or more depending on extent of damage
New gasket approx $165 for the coupe. Sedan probably much less.

Worth doing if you like the car.