Most difficult part is tapping and soldering into the rear speaker wires, I did high level input since it made the most sense. The wiring kit it comes with is satisfactory and includes everything you would technically need but I went and got 4 gauge for power and ground and an in-line fuse, so it took some extra time for that.
I just posted my process in another post but here it is again in case anyone is looking for it, and I actually found that you don't need a addon fuse for the remote cable, the sub seems to turn off after 10 minutes or so as long as you don't open any doors: "I have the FWD S trim with (supposedly) Infinity speakers, no sub. I added a spare tire subwoofer to mine by running L+R speaker wire from the rear to front through the wire channels along the sides of the car (pull the weather stripping and plastic panels). I exposed the front speaker wires where they connect through the harness and tapped them with the wires from the back, I chose front L+R because they are much louder than the back so much better signal, literally have my sub at about 40% and it's very loud, it's the Rockville spare tire one. I ran the power cable along the driver side wire channel and poked through the large rubber gasket beneath the steering wheal area, disconnected the battery, and whatever has the two large harnesses in the engine bay blocking access to the gasket (computer maybe?). Then I was able to fit my hand in there and grab the wire for power. It was trial and error finding which harness had the speaker wires as not much about this topic has been discussed with this car, so I pretty much found other Kia wiring colors, pulled harnesses to see if they disabled the speakers and lucked out with the connections, though did have to reverse polarity on the left because there was phase cancellation, no big deal fixing from the trunk. If anyone needs more detail let me know, I would have loved to have this info when I did it."