I have a factory XF / Zl manual, so I will try and help you. Please bear in mind that those vacuum flourescent dashes may look pretty but inside they are an absolute nightmare! I know this because I have worked on a few. They are very prone to bad solder joints throughout, and due to the way they are assembled (multiple printed circuit boards sandwiched together and joined electrically with ribbon cable) they are a pig to pull apart. The good news is a standard XF dash fits and will work.
Anyhow, I am assuming here that the dash does not work at all ie. no self test or any warning lights...
Some basics : have you checked for blown fuses or fusable links??
To check the power and ground basics for the dash -
1) Ignition - should be a RED wire with a GREEN tracer; 12V approx, with the ignition switch ON.
2) There are 3 earths - unplug the connectors off the dash - the earths are all BLACK - please use a multimeter on OHMS range - not continuity buzzer - I would expect no more than 1-2ohms between these wires and the body of the car (or another obvious earth!).
I hope this is of some help. Please let me know if I can be of further assistance (and if you fix it would you tell us all what it was!).
Best of luck
Kendall.