My suggestion is to buy an iOttie Easy View Universal Car Mount ( http://www.iottie.com/Product/Series/EasyView/Car-Mount
), stick it to the middle of the little shelf in front of the MFD (multifunction display), and attach a Garmin GPS to its 17mm ball mount.
If you don't want the Garmin's charging cord exposed, there is a sticky thread on how to hide it: http://www.siennachat.com/forum/209-mods-projects/6668-diy-hardwiring-gps-2011-sienna.html
We have OEM navigation systems in our 2012 Prius v Five and 2014 Sienna Limited but our two several year old Garmin 2595 units that we use in rental cars are incredibly better in so many ways: they have far more map detail - particularly in small towns, vastly superior voice control and we get free map and software updates four times each year - I just updated both our Garmins today. And strangely, JBL audio/navigation system in our 2012 Prius is far more sophisticated than the JBL audio/navigation system in our 2014 Sienna but it is still not as good as the Garmin.
The only Toyota navigation systems I like are those with Entune (we the premium version of Entune in our Prius) but I doubt that you could easily retrofit one of those since Entune nav units are tied to the vehicle's VIN - I doubt if you could register a retrofitted Entune nav on the Toyota Entune website.
If you absolutely have to have an in-dash nav unit, I suggest you go with an aftermarket one.