The bench needs some organization
.
I luckily figured it out on Sunday. One thing that I did is - I wired the T15 connections from the steering wheel column to the as they should be wired (so basically three separate wires between and steering column unit). That didn't make it work.
Then I looked at the connections again - has multiple 12V pins (one for left light supply, one for door supply, ...) and initially I only had one (the one that has no special function) connected. So I connected all of them. And suddenly the thing came alive.
The next thing that wasn't working were the steering column controls again. But this time I figured it out - there is a bus connection going from steering column electronics unit to gateway. And that one needs to be connected. I always assumed that the steering column would generate the messages for steering controls, but apparently it just forwards the bus to the gateway and that one then does the magic.
So finally:
The next step now is to inject my ESP32 somewhere on the bus. My thinking is just break the convenience bus coming from gateway to cluster and then wire up two CAN transceivers so I can basically selectively let through some of the messages, while modifying the others. The only downside is that then I won't get consumption information on the radio and stuff like that.