I have a gauge, an AEM Failsafe, that requires an (optional) rpm input.
This could also work for certain brands of shift lights that require a tachometer sense / signal input.
Researching an appropriate wire to tap into is proving to be challenging!
I've found the tach/RPM wire at the ECU but from my research a more successful approach is to use the cluster wiring.
Thus I believe the correct wire for tapping the tach/RPM signal is the one of many going into the green plug (i10) at the back of the cluster.
Big thanks to Moker who's successful SpeedoHUD install posts were invaluable for deciphering the WSM info.
Result: I'm fairly sure the correct wire is red/white to A20 on the i10 plug. (second pic of plug, third wire down)
^Plug i10 pins A1-A10
^Plug i10 pins A11-A22
^Pinout of plug i10 (I believe this needs to be read in reverse - flipped horizontally)
^This is the detail from the cluster wiring system PDF that shows the pink RPM wire from the ECU leading to the red/white wire that goes into the cluster plug.
^Cluster wiring system PDF.
I just need to cross check with my MY07 WSM: these are from the MY08 WSM.