Amp/ Speaker upgrade questions

Hey Folks,

So I am in the process of upgrading stock speakers and adding an amplifier for better audio listening experience. I have a 2012 ES, basic setup, non-rockford fosgate factory system. Here are a few questions I have and if someone can provide any assistance, It'll be great.

1. Can I use the existing speaker wiring, tap behind the radio harness to add RCA for amp input and connect the output back into the stock wires?

2. What would be the best place to install the amp? Trunk? Under passenger seat?

3. I bought after market component speakers for the front, do I need to use the low/hi split box or if I just use the stock wring It'll just work fine?

Any help would be greatly appreciated.

Thanks