Setting aside for the moment that I have no definitive wiring diagrams for this car, there are a few options for powering the remote. This is a subject area where I did a lot of iterative discussions with Gemini AI to try to figure out what is best.

Here are some of the things considered:

• Find a source of always-on 12Vdc power. The remote itself is designed to operate from a coin cell battery, so it doesn't draw much current. In fact, it probably draws no current at all until a button is pressed. But the buck converter is drawing a little bit of power all of the time. It's probably not much current, but with an extended idle period for the car (for example, parked at the airport during a long trip), it could drain the 12Vdc car battery. That battery tends to be relatively small in an EV.
• Instead of wiring the momentary switch to the button on the remote, short out that button (ie, always pressed) and have the momentary switch control the always-on source of power to the buck converter. That solves the long-term drainage problem, but it brings in two new problems.
  • When power is applied to the buck converter, it has to go through some ramp-up before it applies power to the remote circuit. I don't know how long that period is, but it could be long enough to make it seem like the button press didn't "take". I tried an experiment to see how long that ramp-up delay was, but the experiment failed because....
  • The remote has a "stuck button" lock-out feature. The remote has some algorithmic detection of the button being stuck in the pressed position. For example, maybe the remote fell between seat cushions or you are sitting on it. When the remote detects that, it refuses to send any radio commands. This seems to be the case for my remote, so it completely rules out this alternative wiring scheme.
• Wire the buck converter to the wires feeding the 12Vdc power socket (aka, the cigarette lighter socket). I pursued this briefly, but the physical layout of the dashboard interior made it really difficult to reach those wires. At least I assume so, though it's possible that those same wires also traveled through the area where I was already working. The other problem is that the power to that socket is shut off as soon as the car is turned off. When parking in the garage, it would be an annoyance to have to remember to close the garage door before turning off the car. It would be an acceptable but not ideal solution.
• There are various 12Vdc signal wires running around. As the name implies, they are only for signalling various things to other components in the car and are not intended to carry more than a tiny current. Tapping any of those wires (assuming I could find them) might not be enough to continuously power the buck converter, or it might interfere with whatever the signal was originally intended to control.
• I was about to give up and pursue the 12Vdc power socket plan when I was lucky enough to notice something that provided an ideal solution (described next).

There is a small LED light that shines into the footwell area. You might have noticed it in the photo of the complete kick panel.

Here's the connector for that LED on the inside of the kickpanel. (The orange thing is just one of my pry tools, not part of the car stuff.)

There are 2 wires (blue and white), and it's a simple "dumb" LED powered by those wires. The LED is part of the courtesy light scheme that comes on when you unlock or open the driver door and that stays on for a little while after the car is turned off. That's a good type of power timing for using the remote control.

I tapped into that pair of wires with Posi-tap connectors and used my multimeter to sort out power and ground. After attaching the leads for the buck converter, I was able to operate the remote while not noticeably affecting the LED brightness.

(After completing everything and being "done" with the project, I had a moment of panic. I had only tested that LED power while sitting in the car in my garage. Why should that LED remain lit while I was actually driving around? It would be quite annoying to have to turn off the car to close the garage door after leaving or to open the garage door when returning home. I tested that the next day, and it turned out the good way. That LED was on the whole time. I think I just got lucky with a 50/50 choice made by the Nissan engineers.)