Hey man, I have a single 10" in mine with the stock head unit. Just buy the sub and box. a decent amp. an amp wiring kit from Walmart and online purchase the adapter from the link below. Run the positive to the battery. The negative to the bolt behind the carpet on the left hand side of the trunk. The remote wire can be plugged in with the ignition fuse under the steering wheel. Then for the line out converter just connect the wires to the speaker coils in the rear and plug the RCA cables in the line out converter and into the amp. Run the speaker wire from the sub to the amp and youll be bumping!! Hope this makes sense
Kicker KISLOC 2-Channel K-Series Speaker Cable to RCA Adapter with Line Out Converter
Hey just my leaving nothing to the imagination 2 cents
Start out by getting a line out converter that supplies a turn on 12 volt signal so you can turn on your amplifier without having to tap into the ignition switch. The amplifier and sub that you get is entirely up to your wallet. I personally have 2 12" with JL 1k pushing them and I love it.
To run the positive 20 feet long cable from the battery positive to the amplifier in the back there is a grommet under the steering wheel that will allow you to go through without having to cut anything or make any new holes. It will need lots of lubrication to get the cable through as it is tight. Make sure that when you run it the cable will not interfere with any moving parts of the steering components, I instantly ran it above the entire steering assembly and tensioner it so it would not have enough slack to ever get close to moving parts. The negative cable you can attach to a bolt under the back of the seat but you will need to stand or metal brush the paint around the area where you want to ground to make a better connection. I personally use NVX cabling for everything since it's good quality and also relatively cheap.
To get signal like mentioned before you can tap the rear speaker since bass is mono anyways you don't have to tap both just one will do. Or if you prefer to take the high road you can tap into the back of the radio by purchasing the appropriate pig tails or wire harnesses for the input and output sides of the radio. A tip to make your life easier, you can use this to power your LOC(Line out converter) as the pig tail does come with a amp turn on cable however if you don't want to run cables you can use the same cables you're using to power your amplifier to power your LOC so you don't have to go hunting for cables and harnesses. Look into PAC LOC as they're a good brand I've used before.
A good place to mount the amplifier is on the back of the seat or if you have the resources you can mount where the spare tire is however this will make your life a little harder if you ever pop a tire.
Lastly if all of this seems like way more than what you bargained for Rockford does make a box with a Subwoofer and amplifier all in one that has a LOC built into it and it comes with either a 10 or 12 inch sub all you would need is the power and ground cables. This is a solution that as long as you're not wanting earth shaking window cracking bass you will be happy with.
Hope this helped and no disrespect to the other person that replied before me.