First of all, DON'T USE AN OPUS WITH YOUR SETUP. That has been addressed in your other thread.
Second of all, I highly suggest planning out your hardware before you start deciding where you're going to tap into electrical circuits in the car.
Third of all, you're talking about your opus like it's some magical piece of kit that will power anything & everything you want in your ride. It has a limited capacity, and even if it did work for your scenario, you'd likely overload it with all you want to power with it.
Again, I am going to direct you to the FAQs: http://www.mp3car.com/vbulletin/advfaq.php
My suggestion to you (and everyone else interested in this hobby) is to research and plan everything. Do it systematically and logically. Planning where you're going to pull power is something that you should do after you've made some hardware purchasing decisions. You shouldn't make hardware purchasing decisions until you understand what all is involved with that particular hardware.
So slow down. Do some reading. Learn some things. Gain an understanding of what you're about to undertake.