Just an update on this thread.

I ordered a Xenon HID relay harness. I just installed it today. As you said Andrew, could not be more simple. There was a great bolt ready to ground the passenger wire as well.

Lights now work with no flickering. So note to all XK owners wanting to upgrade to specific HID systems. Most systems will require a relay to work properly in our trucks (The merchant will not tell you this! lol)