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.