You're right...that model does not have a "tach amp". That system is routed through the
EGR cotrol box in the right side of the passenger footwell. It is also tied into the A/C compressor control relay circuit(if it has a "D-Klima" relay). Since fiddling with the A/C controls used to make it work...I'd look there first. Its rare to have a tachometer or the sensor fail. You can test the sensor with a Voltmeter.
ASE Master Tech
Mercedes Tech 19 Years