Do you have an owner's manual?

 

Should be #14 behind rear seat passenger's heel board

 

Actually, Markus, I was not trying to be a wise a**. The fuse location for the cigar lighter changes on models and years. The JTIS diagram shows the '99 XJ8 fuse to be in the trunk at position #4. For my MY '02 XJ8, it is in trunk position 5. The position you mentioned shows for a MY 98 XJ8.
I believe the manual that comes with the car is probably the best source for the right spot. But, who knows, it might be screwed up too!

 

I understand - I went by the manual for my 1999 XJR, having had to replace my fuse a few months back (clever of them to put the lighter right under a POS cup holder that breaks and spills fluid into the lighter!).