Thank you all for responding and for your suggestions, great!
At the moment I try to remain a bit stubborn (bear with me 😉
The BeoLab 3500 is an electronics product, which ‘boots’ when turned on. When booting it sets for the standard settings. Like for instance basic volume level, LED brightness, etc.
I assume it also sets for a standard audio source/port. So my idea (I still hope it is not a fantasy) is for instance to re-program the settings that get loaded while booting. And by that, set the DIN-connection as standard audio source to ON.
Some products have the ability to change the firmware, or hack it.
I am curious if anyone has ideas about that.