If someone on here bought that shelf, you can get the original factory harness from
Becker 'www.beckerautosound.com' for $64.99 + $15 s&h.
The factory harness terminates at a white nylon retainer that locks into the roll bar on
the passenger side. The retainer is pressed into the roll bar then twisted 90 degrees,
then the harness connector slides onto and locks to it. The speaker harness from the
rear shelf then connects to this.
I went crazy a couple of years ago looking for the retainer when I replaced my
speakerless rear shelf with one that had speakers. No one (Porsche, Becker, etc)
was able to help me. After a ton of research, it turned out to be a BMW part,
available at any BMW dealer. BMW uses it on several of their cars to hold
wiring harnesses to the center console.
Part No: 61-13-1-378-997 2035HBRACKET Lists for $3.67 each