Basically the way it works is the Sony switch takes in both the 'front' L/R outputs from the receiver and the NAVI voice's L/R outputs. It will, by default, just output the front L/R from the receiver. But once the NAVI begins to speak, a 12v signal is sent to the Sony switch telling it to switch to the NAVI voice. What will happen is the front speakers will mute and the NAVI voice will sound. Once the NAVI voice stops, the 12v will be dropped and it will automatically switch back to your receiver signals. The reason I recommended a 4 channel amp is because it's good to have the 'rear' L/R outputs from the receiver hooked up to the amp directly. So even when the NAVI voice is activated the rear speakers can still be heard. Just like how the stock headunit behaves.
The Steering wheel interface is used to retain your steering wheel controls. NOTE: This can only be used if the aftermarket reciever has an infrared remote. Hope that helps.