If you are just wanting to add Bass, you need to know the positive and negative of the rear speakers. You can look at a Metra wiring plug diagram to determine, or if the rear speakers have a (+ or- red dot etc.) then splice in a wire and take them to the Line converter. Run RCA's to the amp, and that should take care of the signal end of things.
Then you will have to find a power supply in the fuse block, or off the back of the head unit that switches on and off with the key, and run a wire back to turn on the LC7 and Amp. Finally run a 12V positive from a battery back to the Amp location.I would suggest 4 gauge minimum. Put a fuse block close to the battery, and don't put the fuse in until all of the wires are in the general area they are going to end up, and a ground from the amp down to a cleaned off spot on the frame(bare metal).
hope this helps I'm sure I forgot something, if I was heading down there in the next couple of days, I'd help you get it going. Unfortunately, I've got to go to McKinney and Dallas.