I haven't done it myself, but know that you need to remove both the front and rear door seals first and then the upper part of A-pillar trim. You can see from the tabs that it basically locks under these three pieces.

 

Hi alexcannas, thanks for the suggestions! I was also thinking that it was necessary to unhook the parts on the right, left and above....but I can't understand if the pieces are linked together by screws, or if they are interlocking (perhaps there is a sort of nail that keeps them still between them)? Do you know something?

 

Remove the front and rear door sill plastic trim pieces first, they are just clipped in, pull upwards, see this thread

https://www.aclassclub.co.uk/threads/sill-carpet-removal-to-get-to-interior-fuse-box.18653/

Then peel away the rubber door seals around the upper and lower B pillar plastic trims.

 

Thanks veeeight, I'll take a look at it, the important thing is to know that they are interlocking and not screw-on. I'll try following the instructions and I hope to succeed, I'll let you know, hoping it can be useful to other users too!