Ok, I found the problem. After changing the wires inside the connector it was obvious that there was nothing wrong with them. So I took the floater out again and inspected it better this time. Underneed there's another small nut that holds a plastic washer, after unscrewing I noticed that the 2 contacts were constantly connected because of a sort of scum caused by the lead (?) in the petrol over the years. After cleaning and sanding away the dirt I refitted and tested with fine result: all works well! That saved me a new petrol sending unit ;-)