Previously, I owned a 2014 Nissan Leaf SL with the Bose system, and by the time the lease was up the range had decreased to the point where it was a little annoying based on my daily commute distance (31 miles 1-way, mostly highway, so it eventually lost the ability to make a round trip without charging it at work), so I never really had an inclination to upgrade the sound system. I ended up replacing it about 5 months ago with a 2018 Leaf SV without the Bose system, and I plan on keeping this car for a while, so I figured I'd make it sound decent.
In any case, my original plan was to keep the factory speakers and just add an amp and a sub, since I'm sort of a basshead anyways. So, I ended up purchasing the following for my amp and sub setup:
Rockford Fosgate P3D4-10 Punch P3 DVC 4-Ohm 10-Inch 500-Watt RMS 1000 Watts Peak Subwoofer
BBox E10S Single 10" Sealed Carpeted Subwoofer Enclosure
Pioneer GM-D8601 Class D Mono Amplifier with Wired Bass Boost Remote
Surge F-4 Flo Series By Surge 4 Gauge 2000W AWG Amplifier Installation Wiring Amp Install Kit
Procraft 2U 12" Deep Equipment 2 Space
I picked the Pioneer amp mostly because it has auto-sensing and doesn't require me to run a turn-on wire.
On the 2014 leaf, I had the cargo organizer, so I considered getting the 2018 version to hold the amp and sub, but the trunk had so much more usable space on the 2018 leaf I didn't want the cargo organizer taking up so much space. The idea was to have the 2U carpeted rack enclosure hold the amplifier, and I would have the option of putting the sub enclosure on top of the amp enclosure or I could keep them separate.
The inside of the 2U rack was wood and not carpet so I initially mounted the amp to it with industrial strength velcro, and planned on securing it with screws after I was done with the installation, although to be honest I've been lazy and it's still held there with industrial strength velcro.
I was also planning on securing the enclosures to the frame, but based on where I ended up putting the amp and sub enclosures, they actually don't move around at all, even with aggressive turning and braking, so again I've been lazy.
Connected the power wire to the battery, put an in-line 100A fuse about 6" down, then ran the wire through the firewall. Spent a lot of time trying to figure out where to run the power through the firewall. There's actually a single point in the firewall on the driver's side (above and to the left of the brake pedal) that has a bunch of wires running through it. I ended up just running the power wire through the same point. Then just put the power wire under the carpet until I got past the rear seat. If you fold down the rear seat you can see where the power cable comes up, but with the seats up you can't see any loose wiring at all.
Pulling up the door trim and allows you to just pop off the B-pillar cover, and that was pretty much all that was required to wire the amp. For the audio input, I spliced into the wiring for the rear door speaker. I didn't mess with the wiring in the doors, the wiring to the rear speaker goes through the b-pillar, so that's where I ended up splicing into the rear speaker wire. If you do decide to use a line out converter, there's some empty space in the b-pillar to stash it, so it's a good spot for handling this stuff. I ran the remote volume control to the edge of the carpeting next to the shifter, so I could adjust it by reaching down.
Rest of the wiring is pretty standard for a amp/sub setup. Power wires hooked up to the amp, along with the inputs, speaker out from the amp to the sub enclosure. The sub had dual voice coils at 4 ohm, so hooked them up parallel to drop the impedance at 2 ohms. Sub wants 500 watt RMS, amp puts out 500 watt RMS at 2 ohms, so I was pretty much done at that point.
At this point, the bass sounded great, but the factory speakers started clipping like crazy at the volumes I wanted to play the bass at, so after a couple of months, I decided to replace the factory speakers with something better.
Ended up getting 4 Rockford Fosgate Prime 6.75" speakers: https://www.crutchfield.com/p_575R1675X ... l?tp=78072
Crutchfield throws in the adapter bracket and speaker harness for free, so it was only $80 for all 4 speakers. The instructions for the 1st gen leafs still apply, and between the adapter bracket and speaker harness I didn't have to drill any holes in the car (although I had to drill some holes in the adapter bracket). The bracket that ships is designed for 6.5" speakers, and I had 6.75" speakers, so I had to do some extra drilling to fit it into the bracket properly. In hindsight, I would've just gotten 6.5" speakers to save me the extra work.
Sorry for the stream of consciousness post. Wasn't planning on posting anything, so I didn't take pictures as I did the work, but it was a fun project for sure, and wanted to share that doing this on a 2018 leaf isn't any harder than on the 1st gen leafs. I'll take pictures tomorrow during the day of the visible components (remote volume control, amp, sub, enclosures).