Wanted to thank everyone for their help and pass on my experience. I put in an underseat subwoofer, the Infinity Basslink SM, which is apparently the same thing as the JBL BassPro SL just rebranded and silver and cheaper.
I reviewed wiring diagrams I found online for the Model X (googled for the service manual), this post
Does anyone have a Model X MCU2 Wiring Diagram?, and compared also to the YouTube video from Light Harmonics (thanks
@TOBASH ) so I knew I was looking for a place to tap the green/green-white wires that go to the right front passenger door woofer. The video where they opened the model X door helped me confirm these were the wires I wanted and happened to find in my 2016 X.
The easiest place I found to get to this was the passenger side kick well where the cables are routing out the car body and into the door. The full wiring seemed accessible under the MCU if you pop out out the cubby underneath it, but the passenger kick well was not hard to get to either. I had to take off trim under the glove box, then take off the door floor trim on the passenger door so you can get to a torx screw holding the kick well trim, gently open that enough to the side that you can pull back the carpet in that area and the wires were then easily exposed as they went into and out of the connectors the wires all plugged into.
It was a bit of a leap of faith here but I found green/green-white wire pair that were well exposed without me opening anything further like wire sheathing, so I put in a posi-tap on both, connected my speaker wire and tucked it under all the trim I pulled off and put everything back together. There is a lot of room for the posi-tap. Those were expensive but I don't see any other easy way to tap these wires--positaps made it easy to secure the wires I wanted to tap into and actually get into them, without exposing more of the trim. I then connected the speaker wire to the sub--it wants both Left and Right source, so I used this once source for both.
I am running power off the 12V outlet using a 12V plug that I've wired to the underseat sub. I'm definitely aware this is not recommended for real amplifiers, but this speaker's specs say it has a max current draw of only 12A. In my testing with a multimeter at the higher volumes it was pulling 7A, so a decent margin under the 10A continuous load (15A peak) Tesla says the 12v outlet can do. I am replacing the amp's 20a fuse with a 10a fuse for extra protection. As 2 other power source options, I'm thinking of wiring it to the spare 12v power circuit I read about in another post (although this has same limits as 12v outlet) but not sure how to actually tap these power wires as they seem plugged into a harness. Is going to the fuse box using add-a-fuse like they did in Teslatap's site any safer? My concern with add-a-fuse was that the wires coming from the add-a-fuse were all 16g or thinner. The 12v outlet plug I'm using has 14g wires so that made me feel a little better...
So I know this is total diy and a few steps here are probably not the best options (using one high level source as the single source when my amp is looking for left and right sources, and using the 12v outlet as power instead of wiring directly to the battery with a fuse), but given it's a low powered sub, so far I still feel it's safe. The whole process only took an hour and half. Sound-wise for $150 I'm thrilled and makes me miss the sound quality I have in the 3 a lot less. It definitely helps fill in the lower end, and as a middle aged dad with young kids I'm not looking for anything more than that!
Edit: will upload pics soon in case it helps