Update: while waiting on my last message to be approved by the mods I spent some more time digging and figured it out.
My biggest issue was that I'd unplugged the "radio amplifier switch control" (p/n 99664520105) when I relocated the climate controls. I'd seen cars without it and just assumed it wasn't necessary, but as soon as I plugged it in and got the RCA connections right it worked perfectly. I've
never pressed a button on that panel, so I'm going to leave it plugged in and just zip tie it underneath the center console so I can keep my cubby where it's at.
As for the RCA connectors, there were two pairs coming out of the head unit side of the wiring harness and one pair from the car-side of the harness. The car-side cables were black and the HU-side were black for one pair (AUX IN) and grey for the other (AUDIO OUT (iirc)). I assumed the black cables went to the matching black cables but the grey pair ended up being correct and as soon as I plugged those in everything worked like a charm.
Once I figured out the user error setup was pretty easy and it was more or less plug and play. I loosened the gauge cluster so I could attach the antennas to the dash (running the cables down through the gauge cluster) and I ran the SIM card dongle and USB cable down by the drivers footwell. My plan is to add a panel-mount USB port to the back of the cubby I added so that I don't have dangling cables anywhere.
My only gripe, like I've seen echoed a few times, is that the mounting solution SUCKS. There are four metal tabs that I guess should click into place but they did nothing on my car. Maybe I cut away too much of the bezel to fit the unit, but it was completely loose once I installed it. I used some 3M exterior panel double sided tape to hold it in place. Works pretty well and was easy enough to install, although I wish I'd used thinner tape as you can see a bit of grey along the top. Going to try to camouflage this a little next. Here's how it turned out for me:
