There were only two versions of tach wiring for that car. If you have no tach amp, you have a control box in the right kick panel(behind the plastic to the right of the floor in the right side of the car). Otherwise,it never worked.This version of the system also ran from a separate wire to the + side of the battery(i.e. at the battery terminal). I have seen these connections forgotten after battery replacement.Look again.
ASE Master Tech
Mercedes Tech 19 Years