Prior owner had tapped in near back of tunnel for XM radio and LED light strips in footwells.

Here is a close up

2002 XJ8. Black is a ground and runs forward to ground point near shifter. White/Blue runs back in wire loom. Don’t know what White/Blue and Black went to. Guessing wired for cell phone which car did not come with. Sound right? Any other ideas?

 

As this isn't an OE wire, there can only be guessing. I can tell you that I tapped into wires in a similar position (on my X-Type) myself to get a signal, which I needed to make the rear camera work. The car came with a SONY aftermarket big-screen-radio, where a rear cam can be connected, but this was apparently too much trouble for the previous owners - it came without the cam. Thus I bought a cheap reversing cam and wired it to the car and to the radio, and now I have the picture of the camera on the radio screen, when I switch into reverse. I think the wire I tapped into was the reverse gear signal.

 

Maybe misunderstood. His add-one were not OE, but the ground and white/blue wire are. Trying to figure original use of white/blue wire.

 

Maybe you find an answer in the electrical guide:
https://en.jaguar-club.net/manual_download.php?id=336

 

Figured it out. Wires to back cigar lighter.