Here's the tutorial, it has its own thread https://www.beamng.com/threads/tutorial-how-to-add-your-own-music-to-the-gavril-vertex.71635/
This mod is one of my favourites, but it has one small problem. On RLS career it fills up the dealerships to the point that you barely see any similarly priced cars, like Covet's and Pessima's. I've spoken to a couple of other people who have also noticed it too. I could tweak it myself, but that isn't going to help anyone else. The next time you work on it could you possibly tweak the rarity values please? I'd love to keep using in career mode. If you want I can go through some other cars files to reference their rarity values in order to work out where they should be..