The following worked for me on a 2000 XK8. It took me 3-4 tries before I got the 'programming mode' to respond. The security module must have parameters on the number of flashes within a prescribed period.

All RF handset transmitters belonging to the vehicle being serviced must be available. If the learning process is carried out without having all the transmitters present, all previous transmitter codes will be overwritten, rendering the missing transmitters inoperable.

Hold headlight flasher stalk back.

While holding flasher stalk turn ignition key to position 1.

Release stalk and flash headlights 4 - 5 times.

Alarm will enter programming mode confirmed by "beeps" from the alarm and flashing LED on gearshift console

Program each transmitter (up to 5) by pressing the lock or unlock button on the each transmitter once.

The alarm will "honk" when you press the transmitter button.

The alarm must honk 5 times to complete the programming. Therefore, if you are only programming one transmitter you should operate the transmitter 5 times. If you are programming two transmitters you will operate one transmitter twice, and three times on the other. For five transmitters you will operate each transmitter once.