I actually did away with the 2 gizmos ( noise filter and condensor) years ago. My tach works fine now without those cracked wires.
The squarish gismo(noise filter) has a grounding signal that goes thru it from ingniter or coil , or both,,, to the tach. As I understand it an inductive signal from that negative wire triggeres the tach. The tach has a + wire going to it from the ECU feeding it juice.
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And , not to forget, cracked wires can allow things to work sometimes and sometimes not.

dave h.
'89 Raider V6 5spd;Aisin Hubs;; Gen2 LS: frt. brakes, rear coils;U.C.arms;R.trailing arms;idler arm; rear LSD axle w.disc brakes ;2 battery system for Dog's fan; relocated ECU; custom bumpers;J.Baker receiver;Conferr roofrack; t-bar crank.