@Goldfinch305 , you base vehicle will be a Ducato x244 with presumably a 2.8jtd engine.
I fully agree with the logical checks suggested by
@Anthony489. In further clarification I would expect to find 3, or 4 fuses mounted above the battery positive terminal. The relevant fuse will be a midi fuse mounted on M5 studs (8mm spanner) and coloured either pink 125A, or black 150A. This fuse supplies the engine bay fusebox via an M6 stud (10mm spanner). The engine bay fusebox supplies the ignition switch, and headlamps etc.
As suggested check all connections for tightness. I strongly advise removing the battery negative connection, before using a metal tool on the above connections.
If the above connections are sound then is there any possibility of past water ingress to the engine bay fusebox? This could cause internal corrosion.
See this post which applies to a Stilo, but the fusebox is similar. Again remove battery negative.
Earth connections have been mentioned, but these seem to be less of a problem than on the x250, but perhaps it could be a relative numbers thing.
The main chassis earth point is below the front of the battery on the RH inner wing in front of the engine mounting. 13mm socket with extension bar required. See photo. Best to apply thin smear of vaseline to cleaned contact faces. Headlamps and ECU are earthed to the chassis, so this earth pont is in the critical current path. Corrosion inside the flag terminal has been reported.