First of all make sure that cooling fan(s) is / are not seized. I can’t remember whether the early Ducato had one or two cooling fans.
I think the relay is the biggest one in the centre of the fusebox after removing the cover beside the air filter.
Some early Ducatos do not have this relay, instead they have a small circuit board mounted within the radiator cowling plastics which could be corroded. This circuit board is still reported (as a relay) by the code that you are seeing.
Get your nut under the bonnet and get your hands mucky for once.