What color for power?

I think it is likely that there are NO "hot" wires in that bundle at all.
You need a wire that has voltage on it all the time that the vehicle is "ready" (running) but not hot when the vehicle is OFF.

 

Not necessarily.
You need to start thinking about longer wires to connect it to where there is suitable power.

 

Maybe tap into the existing power port wires. They are 15 amp circuits.

 

Yes, that's what I was trying to say.

 

You can find a decent place to tap in if you take a look at what is available. Just don't use any circuits that are used for onboard computers, airbags, brakes etc. and avoid low powered circuits. If I was to put one in the very back of the car I would just attach directly to the battery with a fuse, but it would stay on all the time. A relay would be needed to make it switch off with the ignition.

 

IF.....you are going to go to all of the trouble to find "switched" power to trigger a relay, you probably could just use THAT source to actually power the USB device. They don't take much power.

 

Prius 12V outlets

Add a new circuit to your car using an add-a-fuse-adapter

How to Add a Circuit to Your Car : 8 Steps (with Pictures) - Instructables

 

If it's a recent USB device conforming to the USB Power Delivery specification, a separately fused circuit and a relay would not necessarily be overkill.

 

The purpose of a relay for this application would be to switch the power port off when the car is turned off so as not to drain the aux battery. Also, the relay would allow for the additional power port to be connected to the aux battery so as not to place a relatively large current draw on some random nearby circuit that is not designed to sustain an additional draw.

 

Do you really NOT understand that in order to do that, you must find an electrical circuit/wire that goes off when the car is turned OFF to trigger the relay ?

AND....he is not installing a "power port" but a USB charging port which does NOT have a "relatively large current draw".

IF he was installing a multi-purpose cigarette lighter style port, the answer would be entirely different.

 

I really do understand how relays work. The triggering of a relay is accomplished with a low amperage; that's one reason they are used instead of running heavy guage wires for long distances...understand? Are you suggesting it is better to randomly tap into some nearby circuit and hope for the best?
A USB port can draw up to 5 or 6 amps, and that's quite a bit to add onto some of the circuits. Look up Quick Charge USB port; these days many phones require it.

 

Sorry. It didn't sound like it.
And you are right about the best way to do it.
That would be either "activating" a spare switched and fused circuit or using a relay.