I know of a harness kit that enables you to hook up a head unit to the existing set of factory amps and speakers. Unfortunately, those amps won't turn on if you change the resistance of the speakers connected to them(factory speakers are 8 ohm, most aftermarkets aren't). If you plan on replacing all of the speakers, about the only thing you can do is gut the wiring, and run new cable for all of your new speakers. As far as the aftermarket tweeters go, I myself have never seen a kit to mount aftermarket tweets. I'm sure it wouldn't be too difficult to modify the existing setup to accomodate different horns. I myself love modifying things like that. A dremel can make quick work of just about anything plastic. That being said, I am about to go remove the reflectors out of my stock headlights. Good luck!