If you find both of your trunk lights are not working, I had both lights not functioning. The fuse was good and after some investigation I found the microswitch in the trunk latch failed. To access the switch which is part of the trunk latch, remove the trunk floor, then remove the close out panel at the rear of the trunk (4 press in plugs and 4 screw in plugs). Mark around the latch washers so when you re-install it you can put in back in the same location. I ordered a new switch assembly for about $60.00 and installed it; yeaaaa - I have trunk lights. To test to see if it's the switch you can unplug the switch at the connector and use a jumper wire (short) across the female end; If it's the switch, the lights will come on. My old switch tested OK with a volt meter but still was defective. Hope this helps. By the way, the XK8 interior lights are controlled by a interior lighting computer (located in the rear fuse box just aft of the battery), the trunk switch sends a signal to the computer that in turn, sends power to the lights.

Good luck