I have the exact same issue with my 2010 Ford F-150 Raptor with the Sony 700W stereo. Apparently, you cannot connect 2 sets of rear channel speakers through the X930BT?!!
I have the ProClip in my '10 F-150 Raptor.....clips on to the side trim & secures with double sided tape....very solid. I have an IPOD/USB cable that plugs from the cradle into the SYNC USB port. Works like a charm....pricey unit but very well constructed.