Connect your rev counter input to the wire that goes to the NEGATIVE
terminal of the ignition coil.
This wire is normally green on ALL Fords Pre 1995.

That lead goes to the ignition module VIA the inline suppressor filter.


The filter is there to reduce the amount of radiated electrical interference
to the radio in the car.

You could remove it (shorting the wires together) and this will NOT affect
the car in any way but if you listen to non FM radio channels, you will get
ticking and whining noises.