I had a look using google on the installation of the Mediabridge unit I have.... and some posts... as I said the . ..."wires were tucked underneath the bottom of the black center storage compartment, accessible only from the trunk bulkhead."
So... Lift up the bottom of the carpeting ... check the area of the bottom behind the metal crossmembers at the center of the bulkhead.... stick your fingers under and behind everything to locate it.
This is the install guide that has info....
https://www.voxxintl.com/docs/common...500-BMW_OM.pdf