Heya! This is a pretty interesting and niche problem, but I’ll try to break it down and offer some suggestions.

### Understanding the Audio Subsystem
Since your car’s media unit is running Linux, the first step is to figure out how the audio system is configured. Here’s how you can approach it:

1. **Check for Audio Devices**:
   - Run `ls /dev/snd` to see if there are any ALSA (Advanced Linux Sound Architecture) devices. If you see files like `pcmC0D0c` (capture) or `pcmC0D0p` (playback), that’s a good sign.
   - Alternatively, run `aplay -l` or `arecord -l` to list playback and capture devices, respectively.

2. **Check for PulseAudio or PipeWire**:
   - Run `pactl info` to see if PulseAudio is running.
   - Run `pw-cli list-objects` to check for PipeWire (a newer audio server).

3. **Check Kernel Logs**:
   - Run `dmesg | grep -i audio` to see if the kernel has logged any audio-related devices or errors.

4. **Check for ALSA Mixer**:
   - Run `alsamixer` to see if you can interact with the audio mixer. This might give you insight into available channels.

### Recording All Audio Channels
If you’re unsure which audio channels are active, you can try to capture all possible channels using `ffmpeg` or `arecord`. Here’s how:

1. **Using `arecord`**:
   - Run `arecord -l` to list available capture devices.
   - Use `arecord -D hw:0,0 -f cd -c 2 -t wav output.wav` to record from the first device (adjust `hw:0,0` based on your device list).

2. **Using `ffmpeg`**:
   - Run `ffmpeg -f alsa -i hw:0 -ac 2 output.wav` to record from the first ALSA device.
   - If you suspect multiple channels, you can try increasing the channel count (e.g., `-ac 6` for 5.1 surround sound).

3. **Capture All Channels**:
   - If you want to capture all possible channels, you might need to experiment with the channel count (`-ac`) and device ID (`hw:X,Y`). For example:
     ```bash
     ffmpeg -f alsa -i hw:0 -ac 8 output.wav
     ```
   - This assumes your device supports up to 8 channels. Adjust as needed.

### Finding the Right Output
Since you mentioned wanting to feed the audio to a USB card, you’ll need to identify the correct audio output. Here’s how:

1. **Check USB Audio Devices**:
   - Plug in your USB audio card and run `lsusb` to see if it’s recognized.
   - Run `aplay -l` or `arecord -l` to see if the USB card appears as an audio device.

2. **Route Audio to USB**:
   - Once you’ve identified the USB card, you can use `ffmpeg` or `arecord` to capture audio and pipe it to the USB device. For example:
     ```bash
     ffmpeg -f alsa -i hw:0 -f alsa hw:1
     ```
   - Here, `hw:0` is the source (your car’s audio system) and `hw:1` is the USB card.

### Debugging Tips
- **Check Permissions**: Ensure you have the necessary permissions to access audio devices. You might need to run commands with `sudo`.
- **Experiment with Channels**: If you’re not getting useful audio, try different channel counts or devices.
- **Look for Documentation**: If you can identify the media unit’s manufacturer or model, there might be documentation or forums with more specific info.

### Long-Term Solution
If you’re planning to keep this setup long-term, consider writing a script to automate the audio capture and routing. You could also explore using `pavucontrol` (PulseAudio Volume Control) or `qpwgraph` (PipeWire Graph) to manage audio routing more easily.

Good luck, and let me know how it goes! 🚗🎶