Your're allowed, but you need more than just the NAV ECU that goes under your seat. You need a new MFD, as the one without NAV doesn't have enough buttons, nor does it have the inputs in its wireing for the NAV, or at least it isn't documented. You also would have to add wiring for speed, brake, reverse, power, etc. You need to rewire your door speaker connection so that the NAV can intercept it. Instead though, you could dedicate the center speaker to NAV. If you want BlueTooth, you need mics.
Dan, very well put. I think the moral of Dan's post is if you want the NAV that is integrated into the Prius, sell your current Prius and get a new one. However, there are add on NAVS that can sit on the dashboard. But I doubt they can be as much fun as the integrated NAV.
