HW features:
  * DIN male plug connector as the main connector- for power, MELBUS and audio.
  * Atmega328p/Atmega168 as controller
  * Linear voltage regulator
  * Stereo audio transformers for separate grounds
  * 3.5mm jack socket
  * Powered by HU? Maybe a small connector for external power? Both?
  * 4 pin UART + reset connector, small ICSP connector (or just pads) to burn bootloader.
  * Break out the spare GPIOs to control Bluetooth module / connect external buttons.
  * SMD parts for low-height footprint- it should just plug into HU.