How do you get the radio code before replacing dying battery in 2000 Jaguar XK8?

If you have checked the owner's manual (if it is still with the vehicle) and there is no radio code card (looks like a credit card), and it is not there, then you will need to contact the dealer, and provide the VIN # and proof of purchase or title (so that they know you own the vehicle and the radio or vehicle is not stolen).