Mercedes Diesel Tachometer sensor / amplifier VDO Note, This was tested in my personal car, an 83 300-SD, - plugged it in, tachometer came right on.  Of note, these often fail in an intermittent manner, and additionally the contacts can get corroded in the car side of the connector.  Make sure those are clean.

Up for sale is a used Mercedes tach amp out of a 1984 300SD. Mercedes. This unit is what reads the engines RPM signal, and sends it to the tachometer which allows it to function. If you have a dead tachometer on a Diesel Mercedes (pre 1984 models) This is generally the problem. This unit has been tested & functions in an actual car, and should offer many more years of dependable service. M-B part number- 001 545 59 32