Power/USB outlet in trunk

[ArBrnSnpr]

I ordered something like this:
https://www.amazon.com/gp/product/B01N4QWJUO/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1

and I'm looking to install it where the factory 115v outlet is in the trunk. I'm new to the Toyota/Lexus world and I'm not sure if white or black is hot and which is ground and my voltometer is pulling a confusing reading from both wires (which makes no sense).

I know the outlet power is switched from the 115v switch near the steering wheel and I'd really rather have a constant power supply.

Can anyone suggest a good power supply to tap into in the rear right side of the vehicle?

Thanks!

 

[76FJ40]

With the power off see which one has continuity to any ground surface.

 

[BukkakeAisu]

where the factory 115v outlet is in the trunk

Any pics of this? I wired my rear-facing drive recorder by tapping the license plate lights (did't want to run that thicker wire through the door jam and I drive with the parking light on during the day so was a simple solution) before realizing there was another adapter for the rear seat row....but I'm wondering if it's in the same place as the GX.

 

[ArBrnSnpr]

Any pics of this? I wired my rear-facing drive recorder by tapping the license plate lights (did't want to run that thicker wire through the door jam and I drive with the parking light on during the day so was a simple solution) before realizing there was another adapter for the rear seat row....but I'm wondering if it's in the same place as the GX.

Sorry, I don't have any pictures but it's the only outlet on the passenger side in the trunk.