On the x244 and later models there is a fuse board (B099) mounted on the battery positive see attached pdf file. The orange 150A mega fuse supplies the engine bay fusebox (B001), which in turn supplies the LHS cab fusebox (B002), That covers the essential functions, while optional central locking, electric windows, and mirrors, have supplies from the RHS cab fusebox (B101).
If using a tool on the battery fuse board or battery positive terminal, standard advice is to remove the battery negative connection first. I would also take care with the main M6 terminal on the engine bay fusebox.
With the battery negative disconnected, check battery positive post clamp, F70f, and The M6 nut on the engine bay fusebox tightness.
Earthing. In automotive installations the chassis forms the return path for the current, so equally important.
On the x244 model, the battery negative connection descends to the main chassis earth point in front of the RHS engine mounting. The connection is via a flag terminal crimped around the conductor. Internal corrosion has been reported at this terminal, but is rare. The negative cable continues under the intercooler and radiator to terminate on a starter motor fixing stud. This latter connection carries main starter motor and alternator current.
The attached fusebox diagrams are slightly customised, but this is either obvious, or covered by the notes.
Crossing fingers is not an effective substitute for careful examination.