The radio on your 1999 Jaguar XJ8 can be reset by cutting the power to the radio. If you do this, however, you must know the radio unlock code, or you won't be able to fully reset the radio and use it. This is part of the Jaguar's antitheft or theft-deterrent system for the radio. Normally, you don't need to worry about this unless you are replacing the radio or working on the vehicle and need to disconnect the battery for some reason. Resetting the radio will clear your custom presets and return them to the factory default settings.
Open the bonnet on your Jaguar XJ8 and loosen the retaining nut on the negative battery cable clamp using a socket wrench.
Slide the negative cable clamp off the battery terminal.
Wait five minutes and reconnect the battery.
Press the power button on the radio and wait for the system to flash "CODE" on the display screen.
Enter the factory radio code on the XJ8's radio keypad. The radio will reset and unlock.
Take your Jaguar XJ8 to a Jaguar dealership if you've lost your factory radio code. If you've lost your code, the dealer will need to reset the radio for you. You will need to prove ownership of the vehicle.