I just finished an install on my 2013 as well. I got the metra harness and just removed the rear speaker pins and placed them in the spots needed for the steering wheel controls. Took me all of 10 minutes. I noticed you went with rear speakers so yeah you would of needed 2. As for the speakers, you are correct, You have to go through a solid grommet.
So for those not using rear speakers, get the metra or scosche harness and just remove the rear speaker pins from the harness and place them into slots to connect pins shown in the attached diagram. You can use a small eyeglass screwdriver to remove the pins, just insert it into the hole above the pin (when looking into the connector) this will give you a clean install with no factory wires being cut. As for ground, just run a line and create your own.
Pioneer x5500bhs and Pioneer components installed
