Thanks for the quick replies.
This is the best explanation I've seen on stereo installation.
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.
But, if I can just put in the Schosche harness (with red/yelow switch), why in the world does Crutchfield say it won't work with factory Infinity system (w/nav), but will work with the basic system? When I've read posts on stereo installation it has not been clear which type of factory system is being replaced.
If you want to test it, just go to the Crutchfield site and input a car w/o Nav/Infinity and the identical Crossfire w/o Inffinity/Nav system. I can't explain it and neither can the Crutchfield CSR (Customer Service Representative). The system I'm eyeballing is the Pioneer DEH-P7000BT.