Ask HN: Upgrade Car Navigation Button

I recently purchased a Mazda CX-9, and while it has a NAV button, it doesn't actually have Nav (IT's just Nav Ready), I'd like to find a way to use that button for something else, OR even find a way to add a alternate mapping program into the actual head unit (so basically keep everything stock, just change software), I understand I may need to add a GPS receiver to the unit (although I don't know the details on that just yet). Does anyone have any links/tips that would help me go about this project? Will I need to tear up the dashboard to get to the unit and mess with it, or is it accessible via other means?