speedo correction

hi all,due to larger tyres on a gu patrol i purchased a speedo adjuster. its electronic and i need to know which wire to cut and connect it into. from the speedo there are two wires brown and blue,one having 12volts and the other sends a pulse, electrical wise id cut the blue but with cars not all is as it seems (no standards).so im hoping we have an auto elect out there or someone with a manaul cheers rob