This is simple and easy modification to make your OE tachometer to work with signal from ecu tacho output.   Idea is to remove the signal conditioning and voltage suppressing components (capasitor and 1k resistor). And replace the signal suppressing 15k resistor with smaller one. You want to leave the second capasitor (not marked in the picture) in the signal line because it protects the IC from DC currents. Disclaimer: I'm not a electronic guru so don't mind my possibly incorrect terms.   After this mod my revcounter worked beautifully with the DTA ecu. Except the reading was way out. I fixed the reading by adjusting the brass weight screws behind the meter. Now it reads very close (+/- 50 rpm) to true value. There is a bigger error roughly 150 rpm at 3200 rpm, but it is close enough for me.