I am not sure about Mercedes, but a lot of cars have "dummy", or empty fuse slots. Some cars of the same make/model are equipped with certain options/features, and rather than making seperate fuse boxes for them, they just put the same fuse box in all of them. I am not sure what the auxillary fan is though.
1993 Mercedes 400SEL 4dr
2012 Honda Civic LX 4dr
1972 Chevrolet Caprice Kingswood Estate 9-passenger wagon
1973 Pontiac Grand Ville 4-dr hardtop sedan
1988 Mercedes 300SEL (was mine, now mothers)