Seems like someone recently could not get their FOB to program on their X300. I had the same problem today on my 1995 XJ6 after the battery was replaced. The car would not go into the learning mode (hearing the horn chirp) after pressing the valet switch 5 times (with the driver’s door and trunk open). I then opened/closed ALL the doors. Immediately, the car went into the programming mode. The FOB now works perfectly! I am thinking the car thought one of the doors was open?
Hope this will help someone! I did not see this mentioned here or on other lists!
FWIW, a Saab FOB from the same era works IF it is 312 kHz (like the 1995 XJ6). The 1996/97 FOB used 433 kHz.
Richard