Rev counter is ECU-driven on a 2f (and most Digifant systems), so if you've got the 2f engine bay harness you can pick the wire up from there.
It's pin 24 of a GT or G40 ECU, a 0.5mm² green wire - which then splices to a red/black stripe 0.5mm² wire to go into the cabin. The interface is through the bulkhead connector behind the fusebox if memory serves, so might save you running a directly from the coil.