22 Prius Prime - Constant/Ignition 12V Wiring

Since you are already at the rear view mirror assembly you might want to check the power source already available under the cover plate. I recently installed a Fitcamx on my 2021 Prime Limited and power was already available there. Fitcamx provided a swap out cable, but you should be able to ferret out and splice into an acceptable fused source from there, and can go directly to the HomeLink mirror from within the assembly.

 

There will be a constant on wire running to the obd port. The install instruction spec's you need constant on?

And I think most other 12 volt wires will be ignition on, the cig lighter for example.

You can verify with a circuit tester, a gizmo like a screw driver with a light in the handle, and a cable with alligator clamp. Clip the clamp to a ground, then probe with the tip of the screw driver. Light in handle goes on when you've got voltage.