No audio or volume control. What's the problem?

29er Newbie
2015 Lexus GX460 owner here.

I installed the Beat-Sonic LGVIF-CB4W Carplay unit yesterday, and it worked for about a minute and then started cutting out. At first, I thought it was the wifi signal, but after connecting via USB, the problem continued. After another minute of wiggling wires trying to find the culprit, the audio cut out completely. I removed the Beat-Sonic device and reinstalled all the original wire harnesses to the stock configuration. After triple-checking everything, I still do not have audio.

Here's the list of symptoms:
  • No audio from any source (FM, AM, Bluetooth, CD, AUX, etc.), despite all of the sources displaying on the screen and functioning correctly.
  • No volume number is displayed on the screen when using the volume knob or the steering wheel.
  • No audio from cell phone calls.
  • No audio from the navigation.
  • No "beeps" when pressing system buttons.
  • No audio from the seat belt alarm.
Here's a list of things I've done to troubleshoot:
  • Pulled and checked all the fuses, and they look fine.
  • Disconnected the battery for 10min, then an hour, then overnight.
  • Disconnected everything and inspected all the plugs and pins; everything looks good.
  • Entered the secret service/diagnostic menu and ran a system check, and everything returned OK.
  • Entered the secret service/diagnostic menu and confirmed that the radio unit is receiving signals from the steering wheel when buttons are pressed and from the volume control knob on the radio unit.
  • Called Beat-Sonic, and they were accommodating and pointed me in the direction of a cable or connector issue as they have seen similar in the past. No luck so far identifying the problem.
The closest thing I've found to the same issue is this post on ih8mud from a 200-series Land Cruiser owner. It seems the problem was a faulty cable connection.

Is anyone here an expert with Lexus audio systems? If the problem is most likely a cable, which one and which wire?

I found this pinout diagram link, and it seems pretty accurate.
What steps should I take next, guys? I'm completely stumped and don't know what to try next.