Thanks for the great advice. I would never have known where to look and would probably never dared! Anyway, thanks to digital, I snapped a couple images of what I saw.

This nicely padded control unit was tucked in the frame on the right rear, behind the wheel well:


Inside the padding were two loose connectors, one evidently a power supply consisting of three wires:


The other looks alot like the fiber optic connector shown at the Becker Audio site's installation page for Mercedes:


I'm grateful for your help and very impressed by the sharing of knowledge on this Mercedes community site. My thanks!

Glenn