ESP.
Post #20. You can use the 12v switched source on the back of the tach as the 12v switched source to power this filter. Just add a wire to that terminal & run it to the filter. Yes, essentially, the 680 res is connected across the 12v switched source & the coil [-] terminal on the tach. However, rather than separate the 680 res & have it on the back of the tach, I would keep it with the rest of the electrical components & run the 4 wires to their respective positions.