I have mine setup very much like that. I see you still have the factory head unit in there. I have M&T, and I took high level output off the underseat amp for the rear speakers and used a line level converter to reduce levels for the amp. The sound in mine was never quit right, I couldn't get it adjusted so it would just exploded on some songs where the wife would just turn it way down, and the very little reponse on other songs. I surmised that the factory amp was amplifiying the bass, then I was reducing it and amplifying it again to where some notes (standing wave?) would just overkill the car. I just got the idea yesterday to rewire where I'm taking line level INPUT from the M&T amp and ran that to the sub amp in back, instead of taking the OUTPUT and reducing it. Sound is much cleaner, not so muddy and well balanced. I see your pair of 12", and I'm sure you've got 800 watts or more on that, it must just stomp that little car. Mine (Sony 10" ported w/ 800 watts) is huge in there, but has to be well balanced because it was so overpowering. Nice installation, very profesional.
I would love to change the world, but they won't give me the source code