How do you like your Jag? I just traded my 2002 S Type for a 2005 XKR and LOVE IT!! Very Quick (400HP Stock and can be uped to around 550 with few mods) and Ultra looked at! Wow Love the Jags! People tend to think that they cost so much more than what you can buy them for and that is what I like.
I had the same issue with fitting my S type with a Head Unit. I am not sure of the layout of the Head Unit in the 98 XK but my S-Type had to be hard wired and a Double Din had to be custom fit do to no kits being made!
I do not know if you want to up the performance but I got all my performance parts from Paramount Performance in the UK. Great Peaople and the build lots of these kind of cars and have several options from full exhaust to computer remapping with awesome turn outs!
Good Luck with your car!