It's "Infiniti" not "Infinity". If you own one, you may as well learn how to spell it ;-)
Even the "Bose" system is only mediocre. I couldn't imagine buying the base non-Bose model.
The base audio w/o navigation uses a 4-channel amplifier integrated with the AV Control Unit. The best place to tie in to all the speaker outputs is at the AV Control Unit harness.
Front Door Left Speaker (Pin 2/3)
Front Door Right Speaker (Pin 11/12)
Rear Door Left Speaker (Pin 4/5)
Rear Door Right Speaker (Pin 13/14)
The base AVCU is not powerful and it does some signal processing that you will not want if you're replacing the speakers and adding a third party amplifier. You'll want to use something like the JL Audio Fix-86 to process the factory high-level outputs and clean them up into a usable low-level input for your aftermarket amp.
I would absolutely NOT add more speakers. Stick with the doors. Some decent coaxials in the rear and a 2-way component in the front doors is probably your best bet.
Of course, you probably should have just bought a Bose-equipped model, for how much effort and money this is going to take.