I took a photo of the conector where I think I goes but it is hard to know. I am think I have the 10 speaker setup on my Lexus CT200h.
I would not tap into the subwoofer wiring. It has a pretty steep crossover, you will not get frequencies below 80hz. To access the wiring at the amp, you will need to remove the passenger seat from the car, or at least unbolt it and tilt it back if you do not wish to disconnect the seat wiring. The amp cover is held on by 3 small clips, but all you need to do is pry on the cover itself and it will lift up the clips. The bracket securing the amp to the car has two bolts hidden under a flap of carpet, right behind the VIN/Frame port. Once you remove those you can lift up the amp and disconnect the harnesses.My friend just installed a JL Audio ACP108LG-W3v3 for me that has the amp built into the box and he was able to tap into the wires for the factory sub. Because the Amp has a setting for to detect when it should turn on, he ended up not having to run any cables to the front of the car and didn't have to install a bass knob either.
I tried to remove the cover from the Amp under the seat, in case he needed to tap into those cables but couldn't figure it out. It seemed like you'd have to remove the seat to get to it, but i could be wrong.