There are two ways to tackle this, either by replacing the OEM head unit which can get complicated and confusing as you try and reclaim all of the features that come with the car, or by using a LOC. You don't need both.
Assuming you want to keep your OEM head unit you don't need an adapter harness except for 2 speakers. Step 1 will be selecting which speakers you're going to tap, the more convenient ones are usually the rear speakers since you have easy access to their wiring and don't have to worry about fishing wires through the door HOWEVER, it may be better to use them.
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Download a 50hz test tone and play it through your radio at a decently high volume, not damaging to the speakers but loud enough to be annoying, then use the fader to get rid of the front and then the back paying attention to the volume of the tone. The speakers which give a louder response to this tone are the ones you want to tap, but if they're the same then I'd do the rears. It will be obvious if both the front and the rear aren't playing the tone.
Step 2 is actually tapping them, use something like this:
Fits select 1998-up vehicles — connect new speakers to factory wiring plugs
Splice the wires with cables long enough to reach the amplifier and plug the terminal ends into the speaker.
Step 3 is different depending on what you do. If you want to use your amp's speaker level inputs that's fine, but you'll be missing out on bass response. Modern cars lower the bass as the volume gets higher in an attempt to avoid damaging the drivers and the internal HU amplifier. Your aftermarket amplifier won't compensate for this at all, so you'll always have a bass response a bit weak on the high volume end or a bass response that's too strong on the low volume end. The way around this is to get a LOC like the LC2i.
2-channel line output converter for adding amps to your factory system
What this does is restores the bass signal as the volume increases, giving you a much more aftermarket sound than you could get using a regular "passive" LOC or your amplifier's internal high level inputs. I would highly recommend this solution, since you'll be missing out. Wire the speaker wires that you spliced from the adapter to either the LOC or your amplifier, making positively sure that you've got the wires going to the right terminals (L+, L-, R+, and R-), otherwise it can cancel itself out and that's always bad news.
Powering the LC2i is simple, you'll just borrow the power of the amp since it only uses like 25W max. Setting it all up is less simple, but the instructions are clear and there's a million videos about it on Youtube. You're going to want to invest in a multimeter if you don't have one by the way, it's invaluable for setting gains unless you have an oscope or purpose-built equipment for it.