The speedo has been replaced by the previous owner with a beetle one, in order to have the petrol gauge. But the Speedo cable doesn't connect in the same place and I had the feeling that this was the reason why it kept on breaking.
So I decided to replace it with the original one.
I took the Circuit diagram to be sure I would be able to reconnect it.
On the left is the original Speedo, on the right is the beetle one. We can see the speedo cable connection is not on the same place.
While I was at it I opened the speedo and put it back to 3000 Miles. This is the approx milage I have done since I bought the van.