What you'll need is a line out converter (loc). This is the one that I've seen installed the most often ( https://www.amazon.com/dp/B00F0YASQ2/ref=cm_sw_r_cp_apa_i_NgLJCbV0V44CA)
There's install instructions that come.with the product, and there's a few tutorial videos on YouTube as well. You'll still run power to the battery like normal, the only real change is that you don't run anything to the back of the head unit, you just splice into the existing speaker wires.
Seems to be a pretty cheapie LOC. I suggest trying a PAC-AUDIO LP7-2. I've used many of those with minimal issues.
PAC LP7-2 L.O.C. PRO Series 2-Channel Line Output Converter with Remote Turn On https://www.amazon.com/dp/B00F0YASQ2/ref=cm_sw_r_cp_apa_f2StxbWY4QWMW
As long as your vehicle isn't equipped with a DSP from the factory, something as simple as a PAC-AUDIO LP7-2 would do.
PAC LP7-2 L.O.C. PRO Series 2-Channel Line Output Converter with Remote Turn On https://www.amazon.com/dp/B00F0YASQ2/ref=cm_sw_r_cp_apa_H.TAxbMPQ4QCX
This sub enclosure kit is actually very decent for the price. This kit paired with as low as 300W RMS, will keep your wallet and your ears friends.
Just add;
PAC LP7-2 L.O.C. PRO Series 2-Channel Line Output Converter with Remote Turn On https://www.amazon.com/dp/B00F0YASQ2/ref=cm_sw_r_awd_e8Bnvb0F7ZVAV
Stinger SK4681 8 Gauge 4000 Series Power and Signal Amplifier Installation Kit https://www.amazon.com/dp/B007BRB3U6/ref=cm_sw_r_awd_kbCnvb0HH6YR1
Rockford Fosgate R500X1D Prime 1-Channel Class D Amplifier https://www.amazon.com/dp/B00BF6HYDE/ref=cm_sw_r_awd_E-Bnvb15181DQ
A little over budget, but with your type of vehicle, this setup will be more than enough.
PAC LP7-2 L.O.C. PRO Series 2-Channel Line Output Converter with Remote Turn On https://www.amazon.com/dp/B00F0YASQ2/ref=cm_sw_r_awd_3bKPvb5BFCB55
Tried to find some wiring resources but no result. You're gonna have to find some on forums.
PAC LP7-2 L.O.C. PRO Series 2-Channel Line Output Converter with Remote Turn On
https://www.amazon.com/dp/B00F0YASQ2/ref=cm_sw_r_awd_SHXCvb1VRYSC3
It doesn't look like that great of an amp, 300x1@4Ω/500x1@2Ω. It's class AB which is a power hog, 30-40% of your power used goes straight to heat and you'll never tell the difference between AB and D for a sub amp. If you insist on using the amp, I'd just get something like this. A LOC doesn't matter too much for bass so long as it is installed well(ground loop being the worst problem usually). Bring the high level as close to the amp as possible and use the shortest RCAs possible between the LOC and amp, grounding the LOC to the same place as the AMP. For a little more than the LOC you posted you could have 800x1@1Ω or for a little bit more than that 1200w x 1@1Ω. ~~A pisser with the LC2i is you need a remote turn on lead for it, using the stock deck you'll have to run another wire from a switched source to act like the remote turn on for the LOC which has a delayed remote turn on for the amp.~~ The PAC LOC I linked senses the audio then gives you a remote turn on to feed the amp.
edit: after reading LC2i manual, it wasn't obvious from the crutchfield page but it does have GTO sense to turn on when it hears something then power the amp.
I did with my 2012.
I got this:
http://www.amazon.com/gp/product/B00F0YASQ2/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1
and have a single 12" Infinity Reference pushed by an Alpine amp (500 watt @2ohms). http://www.alpine-usa.com/product/view/mrp-m500
The best thing about that RCA converter is that it has remote turn on via signal detection, so I didn't have to run a line to the front of the car. It's pretty well the perfect setup for a stock head unit. I had planned on upgrading everything, but the front sound stage with the factory speakers is not too shabby.
You said "subs". Will a dual-sub box fit into the trunk from the back seat? You won't be able to get it in through the trunk! I downsized to a single sub and it sounds great. What equipment are you installing?