Adding amplifier and subwoofers to JBL system

I know that several other people have made this upgrade before but I haven't found much on the details of how people wired in their add-on amps to the existing system.
The amp I have can accept hi-level (speaker level) inputs. I'm guessing the rear passenger door speakers would be the best to tap into since they are closest to the back of the car where the amp and subwoofers will go.
I am concerned about the possibility of noise creeping into the system since I haven't been able to find any quality shielded RCA wires that will breakout into the speaker lines. Has anyone out there ever used speaker level inputs into an amp and if so what kind of wires did you use? How was the sound quality/noise?

I will post photos of everything when I'm done. I'll admit that this system is a bit "gonzo" for a Prius--800W Soundstream (Van Gogh 800.2) and two Infinity 10" Kappa Perfects.
If I could go back I would buy a smaller amp and 8" speakers. I'm trying as much as possible to keep the trunk unobstructed but the size of the equipment is making that difficult without cutting into parts.
I'm going to build the subwoofer enclosures out of fiberglass and the 0.60 cubic feet of volume needed is hard to come by when trying to mount them in the side cubbies of the trunk.
All I have done so far is the mounting of the amplifier. It took a long time and when I was done, I thought of a simpler way that I could have done it... :roll: ...but at least it looks good and is secure.

Any help on the wiring is appreciated.