Where to find 12v DC Feed

Get a multimeter/test light and start probing the wires to your radio. Even with a basic Radio ,Toyota prewired the harness for more complex units

There is a switched 12v [accessory] [red wire]
a 12v constant [memory] [orange or yellow wire]
and if you look/probe a bit further there is also a 12v reverse feed that interfaces with the radio for the reverse camera.[you will need the ignition on, and the trans in reverse to find this.]

I hooked up a reversing camera to my existing radio.,and while i was in there I also installed a usb charger port connected to the switched 12v accessory.

I did all this approx 18 months ago so the wiring colors may be wrong.[they were Pioneer Radio colors and maybe not Toyota]
Mine was "piss easy" to do because the Pioneer Radio that came with my car had "piggy back" female bullet connectors for the 12v and memory wire.

The most difficult part of the job was running the wires through the hatch to the front of the car.
I recommend using a bit of curtain wire for this [and tape your wires to this]

 

The wiring diagram is another way to find out which wires are which, without having to go poking around at them with meter probes.

Toyota Service Information and Where To Find It | PriusChat

 

There should be a wire running to OBD outlet with constant 12 volt.

 

On the stereo, you will see several multi-pin connectors. Find the 10-pin connector, the only one with 10 pins. Pin #3 will have ignition-switched accessory power.

I can't remember which wire color is used, but you can look up the Toyota 10-pin stereo connector for pinout reference, common to many Toyotas.

 

The OP [and me] are in New Zealand. So there is a 90% chance that his car is a Japanese imported Toyota Aqua.
All the handbooks for these cars are written in Japanese.

The OP has enough smarts about him to try and tap into the cigarette lighter [so the "black art" of wiring doesn't scare him]
people install car radios all the time without using Toyota handbooks [even pro's]

Using a multimeter on a radio harness wont do any harm
So hooking up a 12v feed from the car radio isn't a big scary thing to do. All he needs to decide is whether he needs 12v constant or 12v switched.

 

Doesn't scare people who look in diagrams, either.

But I understand that the "written in Japanese" factor could influence the choice of planning method.

 

I will assume the 12v outlet did not work because you want constant 12v. The outlet is switched 12v.

There is constant 12v and ground easily accessible at the maplight. The diagrams can be misleading as the connector locations are not spelled out on the diagram. I edited and used the following when I needed power for an add on homelink mirror. I used constant power as the homelink only draws 6ma unless being used.

 

my wife is fluent in both written and spoken Japanese and Mandarin Chinese so the handbook issue isn't a problem for me, but it is easier if you are fluent in electrical circuits and go searching for yourself.

But this is a moot point here! @rjparker came up with the best solution. Tap off the 12v constant feed for the map lights. This is only about 6" from the mirror mount reverse camera. and all you need to do is poke the wiring under the head lining.

 

People who can follow a diagram are also fluent in electrical circuits.

'Easier' is subjective enough to probably not be worth quibbling over. Among PriusChat threads where people are trying to solve electrical problems, those eschewing diagrams to go search for themselves often arrive at a satisfactory outcome after a longer series of posts.