You can determine whether you have the GPS 18 or GPS 18x sensor by taking off the left front wheel and and inner fender to expose the sensor so you can read the label. (I haven't done this, so I can't describe it more accurately.) The firmware update applies only to the 18x. But that seems like a lot of work.I sent the cable to Jason Fudenberg for his car, but suggested to him that he find out whether any members of TeslaClubLA also needed the update. If you are a member then you would know him. You can also determine whether the sensor is 18 or 18x using the update cable because the model number is sent as part of the data stream.
Found his email from the last roundup and shot him a note.