An archive of the company behind that tech [1], has this to say:
> Each car radio sends out a signal at a frequency higher than the one it is receiving from the radio station. When a car passes by one of the MobilTrak sensors, the sensor picks up on the signal to determine what the driver is listening to on the radio
And US6813475B1 seems to be the patent behind the tech.
If they ever really could find that sort of signal in the noise of the real world, I've got to imagine that improved tech for in-car radios, not to mention people listening to their phones via Bluetooth and SiriusXM, has rendered it even more broken.
[1] https://web.archive.org/web/20020720075012/http://www.mobilt...