It doesn't have to be in the car/truck to change
Its not looking for a voltage from the speedo input? Being physically in the car/truck isn't really the question.
I have 2 different units (surf, prado) and I spent a little bit of time cleaning them up and painting them. The compass doesn't seem to move in either. I've got them hooked up to a 12V source.
I am pretty sure that testing requires you to run around in circles carrying the FMU and battery.
And posting the video...