Cheap wiring kit - 12 foot RCAs were NOT long enough, I've got some RCA extensions en route. 16 footers should reach anywhere in the trunk. The plastic bit just inside of the kick plate for each door pops out straight up, so it's really easy to get power and RCA to the rear half of the car. Nissan designs things a lot more thoughtfully than GM does, go figure. https://www.amazon.com/dp/B08FRRDZ31

Add-A-Circuit, used on the FR Heated Seats fuse - unoccupied in my Leaf. It's always-on and I'm using low level inputs, so I had to use the remote wire from the sub adapter. The amp is going to draw much less power than the heated seats would, I'm guessing; I'd be driving it hard to draw 30W or 40W per hour. https://www.amazon.com/dp/B08LTJFBHX

Grounded to one of the bolts for the rear seats. Those went back in just fine after adding the noise-canceling material and the subs.

I still need to fine-tune things, but the subwoofer does what I hoped for, adding a little authority and oomph to the factory stereo. My wife says it's a substantially nicer riding experience now; for the outlay of $160 and maybe 4 or 5 hours doing the install work, I'm pretty pleased.