On my install with the harness the CAN bus is used for the steering wheel control. The other box control For getting the RPM, speed, temp, tire pressure etc It's getting the data from the CAN bus from a BT module to the Torque Android app. The 4.2" display is keeped in place with a Y cable, it places the display in line with the Axxess box. The other box communicates with SYNC I think. I'm not 100% sure exactly what each box does but I was told I needed to have them to keep everything I wanted. For example the backup cameras still have the makers with the white wheel positioning.