Unfortunately I am not sure. The wire isn't as thick as the two twisted pairs going to the rear speakers, and I didn't pull much more of the tape back to see where it goes. It is similar to the S550 in that the wiring splits in a 'Y' shape. I believe some of the wire(s) goes to the rear camera...
Was anybody able to confirm the speaker wiring in the rear speakers?
I'm assuming it's the two twisted pairs:
Blue/White + Yellow/Green
Brown/Blue + Grey/Brown
Thought I'd ask before tapping them and testing with a multi meter.
Edit:
I went ahead and installed my sub. In case anyone else...