I just did the same thing you want to do. I replaced all the Mach 460 crap. If your gonna be changing out all the speakers and head unit, you will need to run all new wires and just bypass all the stock stuff. When you put in your head unit, you will need to buy a new wire harness and you will notice the factory unit has two harness. The harness that has all the speakers connected to it, you can just unhook and push it behind the head unit. Just splice the new wire harness into the factory one and make sure you connect the right wires with each other. Then your basically gonna remove the harnesses from the speakers and everything and I just left them in the doors and stuff. Its easier then trying to find where they all go and remove them. Then just run all new wires to each speaker and then to the head unit unless your installing an amp. Oh yeah, you can use either 5x7s or 6x8 in both the fron and rear. You can fit other sized but will need brackets or fab them. Hope this helps, let me know if you have any questions.
