I am by now an expert in this fantastic piece of Italian engineering.
You need to remove both covers of the rotating part of the passarelle. To do this you will need to rotate the passarelle into two positions. Under the cover you will find the connectors that connect the sensors to the wiring-loom that leads to the control box inside the locker. If you are lucky the connectors are still usable but you may find them very badly corroded.
Once you manage to replace the sensor make sure to cover the connectors with self-vulcanising tape to protect them.
The bad the news is that there are a lot of other things that can and will go bad with this thing, most of them very expensive. I spent about 20k$ by now on this thing.