It isn't needed on a 1984 or 85 300D since it simply terminates at a passive "diagnostic port" on the inner fender. That would only be used by a dealer mechanic who has a special sensor to connect there. In earlier years, there was an electronic "tachometer amplifier" plugged into that port. In 1984-85, there is another pickup on the transmission bellhousing which is used for the tachometer input. It routes thru the EGR box (behind passenger kick-panel). I don't know if the EGR box actually uses the signal, or that is just a wiring convenience. I know that you can remove the EGR box and just jumper the pins to send that signal on to the dash tachometer display. I did that when the EGR box stopped working because the OVP relay (above it) failed and I figured easier to rip all that out since the underhood EGR mess is gone anyway (impossible to maintain that stuff w/ no manuals or parts available).

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans