I did it by tapping in to each ground wire that went to each coil and collated them together via the diodes to have the combined signal. Instead of tapping this in to the back of the instrument panel I used the original signal wire from the earth of the original coil. This was on a Datsun by the way but the principal should still be the same. On the S12 I think its the blue wire going to the coil that feeds the instrument, but check.
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