Replacing 2013 Harman Kardon Head Unit (non Nav)

Hi all,

Just purchased a 2013 Legacy 3.6r with the HK 9 speaker system. Fantastic car, but the sound system leaves a bit to be desired. I want to replace the head unit with an aftermarket Nav system that incorporates cell phone connectivity and pandora.

In terms of the installation process;

Can the factory steering wheel controls, blue tooth phone mic, and usb port be utilized with an aftermarket system, and how much modification to the wire harnesses are required? Also, how do I deal with the factory Amp? Do I need to bridge the AMP on/off wire to another wire in the harness, or will have to bypass the amp completely and run all new speaker wire?

If anyone has instructions it would be appreciated. I have low/moderate skills, having replaced many stereos when I was younger but never with Nav/Bluetooth or other advanced features. I am familiar with the wiring harnesses and removing the trim, I just need instructions on how to modify the wiring harnesses to incorporate the factory components as best as possible.

Thanks!