Both my lights are out for the license plate and I'm looking for the fuse box since I changed bulbs and they still don't work.
OBC

 

You don't mention the year of your car, but my owner's manual (2010) doesn't list a separate fuse for the license plate lamps. Is everything else still working?

They must run off a fuse, but I wouldn't know which one (logic would say the same as the rear tail lamps) - I guess you could check the fuse boxes in case any have blown. There's three fuse boxes on the XF - one in the engine compartment under a cover on the passenger side, one in the passenger side footwell by the door, and one in the trunk on the right-hand side above the battery. This will require you to remove a lot of fuses so could prove to be a pain but at least allows you to rule it out.

The only other possibility is that I've read reports of wires in the loom in the trunk lid breaking, which I guess could cause this as well.

 

My guess is the actual fixture. The license plate lights (almost always) work on the same circuit as the tail lights....if you have tail lights, you should have power to the lights...or somewhere along the line. Take a multimeter (or test light) and (with the tail lights on) test the wire to a GOOD GROUND. (Earth if you are in GB). You may have to probe the center contact in the fixture, probe the wire with a sharp pin, look for breaks in the wire, chafing, cuts. Should be an easy fix. I personally like the conversion kits you can buy at your local auto parts store and go to LED....they last forever (almost). Good luck.
Ron