Hi John,
I am wondering what is in the way of seat belt warnings / airbag equipment is fitted to your x230 2.8jtd Ducato? (A Ducati is a motorbike.)
My x244 2.8jtd has drivers airbag, and I believe 2 seat belt pretensioners. The electronic control unit is mounted centre front of the cab floor.
There is an airbag warning light on the dash, but having just sitting in the vehicle without seat belt and turning the ignition ON several times, has confirmed my belief of there not being a seat belt warning light, which would illuminate if seat belt was not fastened.
As in your vehicle the seats rotate and there are no electrical connections either to or under the seats (MH battery connections excepted.)
I have visually scanned available wiring diagrams for the x230 (text is German), and have failed to find any mention of a seat belt warning light.
I have previously owned a vehicle which illuminated a lamp on the dash if a seat was occupied, and the belt was not fastend. This required seat sensors, and a switch associated with the belt latch. The wiring to the belt latch was visible, if you looked.
You do not say what type of vehicle yours is? If an A class motorhome, built on a chassis cowl, then any seat belts could be part of the conversion.