Sub, amp, doing it myself

The factory amp was used for the OEM sub, is that correct?  I'll presume you are replacing that sub if that is the case.  Attach the LOC to the speaker wires after the factory amp to assure getting sub signal.  If the factory sub remains hooked up it will also get a signal which you don't particularly want since you have a new sub.  Disconnect the factory sub.

The LOC converts the high level speaker signal to a line level signal for the amp.  Connection is via RCAs.  Connect left signal to left, etc.  Use the pots, or gains, on the LOC to adjust to define the quality of the sound, and this will take a few sessions to get just right.  In most cases LOC manufacturers say to start at the highest clockwise position and only regress the knobs if needed.  Some say to start in about the center.  I'm an advocate of starting at the highest and turning them back to taste.  Keep this in mind:  every time you make an adjustment on the LOC gains there will be a slightly different voltagle input to the amp.  With any change in input, a complete and careful resetting of amp gains is necessary.

Connect the amp as indicated in the amp's manual.  For startup operation, connect a turn-on wire (18 to 20 gauge is fine) to a switched 12 volt source found locally in the car, or route the wire up to the dash and find it there.  'Switched' is on when the car is on or in accessory position.

..."and it'll be good to go"??   Not until you complete the next 20 steps!   

I'll add this:  factory systems are different from car to car.  The best way to incorporate a sub system into a factory setup is by running wires "wild" to the connection locations first, wrapping bare wire around bare wire, and testing for signal.  That is to say, test before you solder and tuck everything neatly back into place.  If you find that getting the signal after the amp is not correct in this case, go to the front L/R speakers and attach your wires (you could also use the back speaker wires but I prefer the front so that you can continue to use the fade function to reduce the rears without reducing the sub signal).  Testing input signal requires that you set up the amp and sub combo first and have them working.  The "next 20 steps" starts with setting the gain on the amp...