Screw all of that. JL Audio makes something they call the CleanSweep. Basically, you run your factory inputs into this unit, it levels out all the inputs, and outputs them, and uses a separate volume control. My friend, who owns a stereo shop, installs them on many cars with factory navigation systems, and I bet it'd work great in a Gen 3.
http://mobile.jlaudio.com/products_cleansweep_pages.php?page_id=94
The box gets installed in a hidden location, and the only way you can tell it isn't stock is the fact that you have a small **** installed wherever you like to control radio volume. It's a thing of beauty.
You can leave.