It all depends on what you have available, best way to approach it is open both door and look at fuse panels and with ignition off find the ones that are live all the time and then choose one of the remaining ones and test it to see if it goes hot on ignition on and off again.
I would advise against using anything to do with running gear circuits as they go off also on ignition off and try and use things that are none critical to the running of the car.
Piggy back fuse holders are the easiest way off rapping power or you can do it properly and an extra terminal and fuse to the box for your needs.
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