If you want it to sound the best, you're going to need some time and some power tools.
I took my doors off (the entire door, off the hinges), pulled the wiring harness, and drilled a hole into the corner of the harness. Then I fed a speaker wire from the interior through the wiring harnesses, down the flexible tube and into the door cavity. There's no wiring harness to be used if you are swapping up from the base door panels to premium. Link those wires up to the sub in the door and the output of your amp (you'll need to buy this, doesn't need to be ford or kicker or anything special). I don't know about the output of the stock deck but I'm assuming it has subwoofer outputs that aren't being used. Tap into those if you can find them (assuming they exist). If not, you can tap off the front or rear speakers (prefer rear) and just make sure you use a low pass filter on the line or use a crossover to cut the signal down below 100-120hz, or lower depending on the music you listen to.
I've got some pictures of my install on my imgur, ninerone.imgur.com
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2014 GT / Black / 6-speed manual
Airaid CAI / Roush Exhaust / Bama Tune /
Custom Vinyl / Shelby Spoiler