My 1984 300D has no tach amp. That plug was then termed "diagnostic port". Supposedly, dealer techs had a test box that plugged to it, but wonder how many ever did. Otherwise, that front pickup is not needed. Indeed, I tried to remove it since just a gunk trap, but they hard-wired the cable into the engine harness and I wasn't going to snip wires.

Mine is a CA model, so supposedly routes the rear tach signal thru the EGR controller box. I think those engines also have the "rack position sensor" on the injection pump (engine side). Since 1984 fed models route the signals straight to the dash, the cluster might be a different PN from CA, or there is another box nobody mentioned, or the CA cars use the same cluster but don't use some components. Finally, perhaps all 1984 models condition the signal on the cluster, and CA models send it on to the EGR box. Perusing schematics would inform.

Both front and rear pickups are 2-wire sensors. They are a simple Variable Reluctance (VR) type, which consists of a magnet and a co-axial pickup coil. The same are used as crankshaft sensors in many gas engines (see Megasquirt site).

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