Hi Chris,

If the objective is to put a tach into your 89 cluster it's MUCH simpler
than putting the circuitry into the 84.  You'll need a FCA Flexible Circuit
Assembly, that's the blue plastic connector sheet that covers the back of
the cluster which has the Tach connections.  The wiring harness already has
the tach wire included.  The harness plugs into the cluster.

Then just remove the clock system and add the tach in it's place.
Mechanical exchange.  Far simpler in my opinion and it retains all the
sensor stuff you seem to want.

Cheers,

pensioner