Even though the Bullitt sounds amazing on its own, I still think I need to upgrade the sound system. I've seen Bob's thread about changing out the head unit; has anyone else changed any of the components out? I've done a few installs myself in the past but how difficult is it on this car? I'm looking to change the speakers (front and back) first and then the head unit later on. I will be keeping it simple, no amps or subs just yet.