The backlights need a 7V feed, which you can get by picking up a 12V feed controlled by the ignition and using a buck converter (costs a couple of quid). The very long thread about replacing the NIT has some more details. I’ll see if I can post a link. Really not that hard and rewarding once it works.
Edit: Here is the link to where the discussion starts about the 7V feed:
https://www.sportsmaserati.com/index.php?threads/4200-radio-satnav-upgrade.31574/page-10#post-833015
Hopefully this will take to the correct part of a very long but informative thread about everything to do with upgrading the sound in a 4200/Spyder. There is a link to the Maseratilife thread that explains the 7V feed in detail. Happy reading!