USB AUX adapter Volkswagen CD changer emulator — help with firmware and customization

[onatskiy-sergey]

Hello!
I have a USB/AUX adapter for Volkswagen that emulates a CD changer.
Currently, the device is unstable: sometimes it only starts after 10-20 power cycles.
I took the adapter apart and found an ATmega8 inside.
I want to reflash it to:
Fix startup bugs.
Add support for the AU7850C chip (responsible for USB/MP3).
❓ Questions:
Which sketch/firmware is best to use as a basis for the ATmega8?
Would code from ATmega328 emulator projects work?
Has anyone adapted similar code for the AU7850C (track control, status, play/pause)?
ℹ️ What's already known:
The protocol for communicating between the head unit and the CD changer is Panasonic CDC (VW/Audi).
Logic: the head unit sends commands (Play, Disc1-Disc6, Scan, Mix, Seek), and the microcontroller responds with 8-byte frames (status: disc, track, time, status).
There are open-source projects for Arduino (ATmega328),
and other implementations.
🙏 I'd appreciate any help: code examples, ready-made sketches for the ATmega8, tips on adapting to the AU7850C.