Outer Limits 4x4 Board

We are attempting to fit a Jaycar speedo corrector into a GQ. The Jaycar unit has a 12v ignition wire, Earth, and sensor in and out. Sounds simple enough. The speedo sender appears to have only two wires. We have used both of the speedo wires. With one the unit gives no reading and with the other it reads as the speedo did before with no adjustment possible.
Any suggestions on how to get it working properly?

Seems to me that you have hooked it up OK ecept for the sensor cabling. Sounds like the Jaycar sensor goes between the OE sensor and the speedo.

Find the wire in the car that goes from the sensor to the speedo, cut it, then join the end that goes to the sensor to your 'sensor in' and the cut end that goes to the speedo to your 'sensor out'.

Hope this helps.