Hello, I hope the following assists you.
The Mondeo has a driver and passenger seat belt minder, these are integral to the seat belt buckle not the seat belt. The wiring to the actual seat belt is for the pretensioner that forms part of the SRS. If the belt minder keeps going off when driving then check the wiring under the seat, check both seats as the passenger seat has a mat in it that senses if it is occupied, it can cause false alarms if heavy objects are sat on the passenger front seat.
It's common for the wiring under the seats to become damaged, you can also unplug the under seat connectors with the ignition off and check the wiring circuit operates correctly as the belt is fastened and unfastened, make sure you trace the correct wire as there are seat airbags that you don't want to accidently deploy.
Let me know if I can assist further and I'll reply as soon as I'm able to.