In modern cars, lots of stuff gets constant power. FSM should have diagrams which will be helpful. If it's something being wired in-cabin, you can just hook into the ignition switch wiring, which has a constant 12v supply on all but the weirdest cars.
And yeah...I'd love a plain white plate...but that's as close as you get here. Idaho has some UGLY a** PLATES. Look:
https://www.accessidaho.org/secure/itd/ ... lates.html
This one is BY FAR the worst plate I have ever seen on a car:

It's a hundred times uglier in person. The sky is vomit-pink. It's eyebleeding.