I will take a stab in the dark on this one. The 1987 300 SD is the mechanical/electrical cousin of the first generation 124.

I looked into the circuit diagram for my 124 and it appears that fuse C is shared between the radio trunk amp and the central locking pump.

Look closely at the fuses, sometimes they split at the 90 degree bend between the cap and the strip, so they look good but can really be blown.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech