Sits underneath the dash on the passenger side. Has 4 wires, one of them goes to the -ve trailing coil, and I suspect that one of the others goes to the tacho but couldn't get in there with a multimeter without pulling out the dash.

If it's hooked up to the tacho, can I just get rid of it and run a wire from the leading coil -ve like the stock setup? There's another mystery wire I'm trying to source that runs from the trailing coil -ve to somewhere under the dash...