This is a plastic rod that goes inside the automatic shifter of the bmw e39. This does not come as a spare part. Because it's a plastic piece it's normal for it to break at the tip so the reverse protection is lost, and you can go from drive position straight to reverse without pressing the safety button, if the tip is broken like mine. As there is no spare part, you need to buy a whole new shifter knob, which OEM is really expensive. In my case the shifter know was original and in good condition, so I decided to copy the original piece in Inventor, eyeballing the tip that I had broken using reference images from the internet. In the second iteration I made the piece fit perfectly and worked as a charm, regaining the reverse safety lock. You may need to sand/file a bit the hole where the metal rod from the button enters the platic rod, it needs to slide freely. I printed it in PETG. Any question or issue about the piece or how to replace the rod please feel free to contact me.
This video helped me take out the rod: https://www.youtube.com/watch?v=gVx7Hvbgh_A
Final assembly: https://www.youtube.com/shorts/1Upv69RdYBI
Part installed in car and working: https://www.youtube.com/shorts/sZHz14vGN8s

In the image you can see the original part in dark grey and the 3d printes one in silver/light gray.