I will type this out for the 300000000000668684873276347th time.
The signal from the sensor to the speedo head is a sawtooth (AC) voltage +/-1 volt. The speedo head converts that to a square wave PWM 0-5V signal for the ECU (and every other CU in the car that looks at speed). You have to set the Jaycar jobby up for the type of signal you are correcting, and you can't ever try to direct connect the sensor to the ECU, because they just don't speak the same language.
The Jaycar one, I think, works well on AC signals and not on PWM signals. If you have a need to convert a PWM, the smart approach these days would be to program an arduino to take in, modify and spit back out. Would only take a few dozen lines of code and possibly a variable resistor to allow you to adjust the effect without reprogramming.