Never did find it. Apparently there is none. Took it to my mechanic and they replaced the camera which had failed. He said there was no box.
The bill was $950. $350 for camera. Also included replacing the hot air furnace duct under the refrigerator. The factory had cut it too short so they didn't bother to hook it to the furnace.
The multiplexer on my 2015 MBS 24R is behind a small panel above the fuse box just to the left of the driver's left knee. I had to remove a screw on the bottom of the panel. The multiplexer was secured with a wire tie.