The factory Infinity system sends Left/Right speaker level outputs to the factory amp. Those left/right outputs are split so there are 2 identical left/right inputs to the amp. One of those inputs goes to the door speakers, the other goes to the rear speakers.

If you replace the factory nav head unit with the Pioneer and connect the wires in the Schosche harness the same way the factory head unit did (including swapping the power wires and connecting the remote amp turn on wire), you won't need to bypass the factory amp or run any new wires. There are plenty of great posts here with instructions how to connect the wires to the Schosche harness. You'll end up with the same setup -- Front L/R speaker level outputs from your Pioneer getting split up and going into the factory amp and on to the factory speakers.

It gets more complicated if you want to bypass or replace the factory amp, or replace the rear speakers. That would require running more wire and tapping into other existing wires.