In case it helps anyone else in future, below is the final wiring diagram for a Model S w/premium audio. For me, the most difficult thing was getting the panel off below the steering wheel (no real trick, just a plastic pry and brute force), and then dealing with all the wires; I'd already used the space for all my radar detector wires and speaker. finally, wiring everything into the amp at the back of the frunk cubby required me to bend in ways I no longer bend.
I'm still messing with the settings; so will reserve judgement. During set-up, I noticed that my front right speaker(s) (if I move the speaker nav all the way to the top-right) sound like an AM radio, so I think they might be blown. I don't know when this happened; the SC worked on the passenger door a few weeks ago, so it could be a lose connection. Either way, it's nothing to do with me, I wasn't over that side, but I might have to prove that! Overall though, the sub appears to add a good amount of depth to the audio, without being boomy and OTT.
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