The factories making these cameras are constantly changing the guts. So I always recommend that you test your camera powered from 6 Volts to see if it will work, since many do -- even when the vendor's specs say "12 V".
Use batteries or a 6V AC adapter and connect to a TV or monitor. Then compare the picture when powered from 12V, to make sure it's not getting dimmer or noisier when powered from 6V. Some cameras do need just a little more to work properly (7 or 9V).
For a camera that works from 6V, its easier to power it from the HU's 6V camera power pin.
Also, before you start drilling holes, cutting/splicing wires, etc., test-fit the camera in your car connected to your HU. You want to make sure that where you plan to mount the camera will result in an image that is right side up and angled such that it provides the view you need (not looking up or down too far).
See my
Subaru/Toyota rear camera info page for details and additional tips.