When the reverse lights are turned on, it powers a PIC in the trailer that's been pre-programmed with the trailers information in EEPROM (no iButton). That pic modulates the lights, which sends the data to the cab (via current) without the need for any additional wires.
I see what you mean what a clever way to do this =)
But i still dont get it how the cab would be able to differentiate between two trailers
as with your earlier explaination

That pic modulates the lights, which sends the data to the cab (via current) without the need for any additional wires.
With the brake/Reserve pressed the input to the cab would be high all the time as the lamp is ON