Just use a TomTom anyway.
My 2005 Lexus RX300 has factory-fitted satnav but first it's only DVD-based so can't add anything else in (like POIs, speedcams etc) and secondly, the newest DVD available for it is 2011, so well out of date too. The touchscreen display also gives access to controls for climate, phone and lots of other stuff so I wouldn't dream of removing it, but I use the satnav map display as a sort of 'rolling screensaver' and have a TomTom attached to the screen for real navigation.