All of the later MB cars use the Nippondenso compressor with the rpm sensor in the rear. There are a few other cars, Saab or Volvo that also use the same type sensor system but I don't think are interchangeable. Because of location constraints etc it is about impossible to install any other than the OE compressor. Earlier (1984-86) used a GM R4 compressor and 1984 used a Nippondenso without the sensor at all but would not work in your system as it is looking for an impulse to engage the clutch. If you still can not find a sensor replacement try calling
Hancock Industries, listed in the Star mag if you are a MBCA member or contact the Parts Shop.