I don't even own a Civic yet but I was doing some research on the audio setup on the Si and I found a wiring diagram somewhere else on the site. I spent the better part of a morning trying to figure out how the front tweeters were wired up. I found quite a bit of information on here but no one seemed to know for sure. Anyway, the first picture is for the Si and EX systems (with the amp) and the second is for the non amp systems.
This is the definitive wiring diagram, I know for at least the 06, I'm assuming it will be the same for all model years. The only thing that I'm not sure of is if the crossover resides in the amp or if it is a passive setup. I saw another thread where someone showed a resister on the tweeter itself so that may be our answer. The amp may be sending out a full range signal to both the tweeters and the speakers in the door. Can anyone confirm this?
Anyway, SI diagram (coming from the left is the radio and on the right is the amp)
base system:
