i had a question about the factory head unit. i have installed several sound systems on mustangs and other cars and on my other mustang i put an aftermarket head unit in before i installed subs. with my new GT it has a factory haed unit in and i plan on keeping that way. how hard is it to hook up an aftermarket sound system to a factory head unit? is there anything that i need to know or need to get for installation? and also what about the RCA jacks? thanks