There are two ball joints involved in the steering mechanism of my under seat steering recumbent.
One of them ripped out of its socket:
The dirty grey-green part inside the end of the bar is made of rubber and is supposed to hold the steel ball on the handlebar. The rubber part has the number "010-0000-017-008" on top and "12" on the bottom. The metal piece surrounding it has an "R" stamped in. According to a comment the same part is used for the self-leveling suspension adjustment of Mercedes 1999 E320.
Here you can see the intact mechanism:
And here an overview with the whole bike visible, just for better illustration of why that problem is even bicycle related. :)
I tried pushing it back on by hand, then using pliers but in vain. Possibly there are larger pliers which can apply more force, but I already applied quite a lot.
Just for the curious, how this happened: The part where the ball joint fastens to the handlebar can be moved along the handlebar. I moved it out, this means that small movements of the handlebar result in much larger movements of the fork, so the steering is more responsive and smaller curves are possible before the handlebar hits the thighs. This also means that there is nothing preventing the fork from turning more than 90 degrees especially when no thighs are preventing the handlebar from turning, for example when pushing the bicycle by the seat. That's what happened, the fork turned so much that because of the steering axis angle pushing forward resulted in a force turning the wheel even more, until the ball joint snapped and the front wheel pointed backwards.