Cruise Control on a 300TD W124 from 1993

Hi, I recently bought a MB 300TD W124 Sport Line from 1993, and the cruise control does not work. In my MB repair books, I cannot find a wiring diagram for the cruise
control, and I do not want to change the unit attached to the pump, if the fault is somewhere else. Is anybody able to help, e.g. with a wiring diagram and preferably with a hint on how to do problem determination on the cruise control. - Or do I have to give up and leave this to the MB garage?
Thanks in advance.
/Lars from DK
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