I am not too crazy about line level outputs.
I've had done what you are asking about in my GMC Yukon, with great success. I am an average backyard car audio installer, but wanted a professional job done and had a shop install it. Anyway, they "relocated" the factory radio. It's hidden in the rear. I had to buy some other module too to make it all work (and keep the factory chimes and stuff.) I don't have navi in that ride, but it works the same way in the GMCs if you have navi I believe. This might
be an option for you. Maybe they can move the factory unit to the trunk??
Here's a link to my truck: