I just checked and the RSES is not covered in the wiring diagrams because it is a port-installed accessory and not from the factory. So, I can't be sure, but a well-educated guess is the FM modulator is in the ceiling unit. The FM modulator sends an FM radio signal from the ceiling unit to the FM antenna of the dash radio. The antenna picks up the signal and plays it through the dash radio on a specific FM station. Does the FM band of your factory radio work and is the antenna hooked up and working properly? The FM modulator in the ceiling unit might not be a discrete, separate component. It might be just built into the audio circuitry on the control boards for the RSES that are in the ceiling. The wireless headphones and headphone jack are not part of the FM modulator, but it's good you checked to see if there was in fact an audio signal being produced from the RSES by using the headphones. All that being said, I really doubt there is a way to separately test/troubleshoot the FM modulator. Finally, there is role for the remote in the use of the FM modulator but I assume it's working properly with fresh batteries.
