With the 4 stereo inputs, the LC8i is very versatile in routing and summing the signals from your factory head unit. With so many options, it can get be a little confusing.
***Special Note about summing***
Summing mode can sum all channels to the MAIN output. It does NOT sum all channels to ALL outputs.
When summing mode is engaged on a given channel, it does NOT cut off that signal to that specific channel output.
Below are some diagrams of typical scenarios of how you can setup your LC8i.
Click on the image for a larger version
LC8i product page here