I installed the Garmin Mechanic and Garmin nuvi 1450 with Bluetooth in my 2012 Prius Three, it worked well. When I upgraded to the PIP it worked for a while then it kept saying the vehicle is off after 10 minutes of EV operation. Eventually it just stop working (ie communicating). I don't believe the problem was the GPS because the lights on the GM never even got to the point where it was trying to communicate with the GPS. I ordered a replacement Garmin Mechanic and the same problem, not working. Amazon took the both back with a full refund. The first one didn't even have the full packaging. I love Amazon. Everything in the PIP seems to be working well. It will be interesting to see if they find anything at the 5000 mile service. I don't want to bug them about it. If they find something that needs fixing I don't want them to thinks it was because I attached a "foreign object" to "their" connector
Have you dropped Garmin's support a note about your issues? Even if they have no solution, enough problem reports could cause it to show up on their radar. Did it work ok if you turned off EV mode? On the Scangauge II, it had vehicle type selections and you had to select hybrid (on my Gen 2 Prius), otherwise the SG will shut itself off if the ICE turns off. I wouldn't be surprised if Garmin had done no testing against a PiP, since they're not really available outside CA and CARB states. See Facilities | Garmin | United States.
It ended up not working at all. Before that I didn't get a chance to try HV only. Yes, it asked if the car was a hybrid. Good point on the PIP. I get excited about technology but after using the Garmin Mechanic for a few weeks on the non-plug-in Prius it seems there wasn't really that much of interest being displayed so getting my money back was the best solution. Coolant temperature and RPM were interesting but that was about it. If it had worked I would have kept it, perhaps the error messages would have been worthwhile someday.