Roger,

I di not remover the factory speakers that are in my rear deck so I really don't know the details of how to get it apart.

You will have to take out the rear seat and take off the rear deck trim to get to those speakers from the inside of the car.

Here's what I would do. Go to your local MB dealer and talk to the techs. They will tell you how to do it the right way. Mine were very helpful when I was working on my car.

If you want to try yourself. Pull up the rear seat bottom. There are two latches in the front corners. Then take off the seat back. There are two or three screws at the bottom. You are on your own from there.

Sorry I don't know more.
Good luck