Central locking pump on the passenger side...this is in my 93 300E, a pic taken documenting a stereo install:





There is nothing on the driver's side in mine, so perhaps that is a factory audio amp in yours...mine reside in the trunk:

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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