[AKGC]

There is a "power on" signal wire that will need to be connected to the stereo.

There is something called a PAC interface kit that is recommended, but not needed when wiring am aftermarket stereo to an OEM amp.
It basically lets you lower the gain on individual speakers wires, going to the amp. Aftermarket stereos usually have outputs for wiring directly to an amp, reason why I said the PAC kit isn't needed.

 

[AKGC]

I found a harness that is supposed to work with the stock amplifier. It’s about $180 for it. I’ve also read that you can wire power directly to the amp but I dont have the wiring diagram for the head unit

IIRC there isn't anything different with the standard harness connection and the infiniti/JBL one, except for the extra wire for the amp.
For the 2010-2012 you would just add the wire/pin in question and wire everything up.

But looking at the pinout for the 2013, it seems that the "power on" signal is just wired to the ACC of the stereo... weird.
I could be wrong. Posting the pinout so fresher eyes can point something out that I missed.

Pinouts

Apparently there are 2 different type of amps