If you open up the 940 which was the only x40 device to have mp3 player AND FM Tx you will find one of these:
If you open up the 950 guess what? that little PCB is missing, sure the slots in the case where the PCB would clip in are present but the PCB is missing.
Just in case you haven't worked out what the above PCB actually does - Guess what? yup thats the FM Tx unit so there is no way to get the FM Tx working on the 540, 749, 550, 750 or 950 as the hardware is missing, if mp3 player and FM Tx is important see if you can purchase a 940 from somewhere in Europe - Mike