i just hooked some thing like that up in a mates car not HKS but same sort of deal
Grey - Handbrake switch, goes to the handbrake switch the wire is next to the handbrake. take off all the centre console and join it to the wire comming off the handbrake.
Black - Earth, a good bare metal point.
Blue - Speed Wire, goes to wire #53 on the ECU harnes. The wire is yellow-green with a silver trace.
Brown - Rev wire, once again on the ECU harness how ver im not sure which wire im sure someone on here will know