This is an old revision of the document!
HW features: * DIN connector (plug or socket?) for audio * Atmega328p/Atmega168 as controller * Stereo audio transformers for separate grounds * 3.5mm jack socket * Powered by HU? Maybe a small connector for external power? Both? * 3 pin UART connector, small ICSP connector 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.