you should install the LOC where you can easily get to all four channels (or however many you choose to run). I would recommend behind the headunit, assuming you are keeping the factoy one. If youre going aftermarket you will not need a LOC as your headunit will have RCA preouts which ran be run directly to the amp. also some amps have high level inputs built in, making an LOC unnecessary. but you still have to tap into the factoy wiring if using an LOC or the amp high level inputs.
Im new around here, i dont have a wiring diagram readily available.