A short answer to your question is that the fiber optics connector is split open and the "other" fiber optics cable is installed in the place of the existing one of matching lens color. If the seperate one, labeled PHONE IN has a red lens, then replace the existing red lens cable, or the white or clear one, if that what the seperate cable is. Simple, right?
The problem is of course in the trunk or rear of the car, this has to form a optical loop and come back to the radio.
I did have a diagram all made up of this system, it's quite complex and of course MB expects it to be in a programmed order.
There is some "spare" cable in the trunk also you'll notice. I assume you're installing the voice recognition unit also, I thought there is an unused cable which has to be utilized when the VR is installed also, and one of the cables changes function.
Once you get this all figured out, the coding in the radio will also need to be changed using a shop computer (SDS) to make it all work properly.
But the most critical thing is that the fiber optics cable all has to run to the components in the proper order.