The line out converter as well as the high level inputs accept high level speaker wire inputs. My 2 points were:
1. no reason to pay for the converter when most amps have that feature built in.
2. if you grab a pair of inputs off the head unit before it goes to the factory amp then even through they will be speaker wire, they will be low level. just grab the rear set of wires before they go to the factory amp. I would even suggest taking the set and running it through a passive 2-way crossover. then send the high output to the factory amp which will then power the 4" rears which will benefit from the bass filtration, and run the low output to your amp for your sub. Since this will be a speaker wire, you can either run it to the high level inputs on the sub even though it isn't really a high level input, or take an RCA cable and cut one end off and hook the bare wire end up to the crossover. I think that the outside connection of the RCA is possotive, but I am not totally sure. If the crossover you get has RCA inputs and outputs, just solder an RCA plug onto the wire from the stereo. It should be pretty easy to sort out, put feel free to ask any questions if I was not clear.