There should be a paper template residing in the fuse box cover containing the lettering/numbering chart for each fuse location.

If that is missing, you will also notice that each panel in front of the rows of fuses is embossed with the number/letter of the corresponding fuse.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car