I saw today on Tom’s Hardware Guide that he did an article on car PCs. Those are sooo last month!! Only Joking!
I did see that a company in the USA called Blitzsafe makes adapter cables for head units without AUX in by fooling the unit into thinking it has a CD shuttle installed, and giving female RCA leads to plug whatever you like into. That would have saved me some time and stress!! Doesn’t look like they make Opel or Vauxhaul cables, so I wouldn’t have been in luck.