It is in the rear adaptor plate on the back end of the engine. Down on the side edge. The harness
comes up and connects to a 2 pin plug at the top of the firewall in the center, above the valve
cover.
It is where #233 is pictured here:
http://www.detali.ru/cat/oem_mb2.asp?TP=1&F=123193&M=617%2E952&GA=722%2E120315&C
T=M&cat=143&SID=01&SGR=015&SGN=04
You need to check it with a multi-meter that does AC voltage.
Unplug it and check the voltage at the 2 pins of the sensor side of the plug. You need greater than
0.5 volts AC ~ at idle, should be around 4 volts.

If you do not have it, either the sensor is bad or the flywheel is bad.

If you do have the voltage, then you need to start tracing the signal. The rpm signal goes from the
sensor to the EGR control unit mounted in the right kickpanel. From there it goes to the tach and to
the klima relay mounted in front of the fuse box. The EGR control unit is controlled by an
overvoltage relay and the 10 A fuse may be blown. It is under the dash on the right side.