I report that I have successfully found the interface unit.
It should arrive soon.
All in all, I've spent around 60€ in total on all components so far.
AP800's are not available anymore. The current model for this car would be the AP900Ci. Those go for around 350-400€
The effort and time I've put into this so far, is well worth it given how much money I'm saving. I really can't complain
I might document the installation here, once I have time for it. Maybe.
Of course, I first need to figure out the wiring 😄