I see all the threads with people adding an amp and sub to their factory Bose systems, but not very many people who completely replaced their front stage and added a sub. I want to replace my fronts with an aftermarket component set, add a sub in the rear, and an amp to power them both. What will I need to successfully hook up an aftermarket amp/amps to my OEM nav HU?

 

For the speakers, subs and amps, you can DYI if you can understand section AV in the service manual. But if you don't have any clue after reading the service manual, bring it to a professional installer to get it done right.
Why would you want to change the head unit in the G?

 

I don't want to change the HU, hence why I'm asking what I would need to hook up an aftermarket amp to my factory nav.

 

Sorry, my bad. I didn't read it correctly.
To install the amp, cut the audio signal wires that coming to the stock amp then rewire them to the aftermarket amp. Same thing for the speakers.

 

That's it? No other devices or harnesses to use?