Why does it say on Crutchfields web page for ever head unit that you have to buy a aftermarket amp when you replace you factory radio. I have done this a thousand times before but I don't want to buy a aftermarket radio and have to buy anything else but a wiring harness for it to work with my existing MACH audio system. Basically, can I keep my MACH system for now and replace the factory radio without any other modifications?