If Nissan refuse to warrant your tail lights because you fitted an ARB Deluxe Bull Bar, I'd be laughing all the way into the Supreme Court with the case.
Talk to your dealer, and the service manager. If they're halfway reasonable, they'll tell you, like Krafty points out above, that if the mod that you made contributes or causes the failure, it won't be covered by warranty, but if they're unrelated, then Nissan have no problems covering it.
What IS related:
* Bodgey installation of snorkel stuffing the air box and ruining it. I doubt that will happen. Safari actually test their installations, I've heard, so that choice is a good one.
* Non-certified bullbar installed and affecting airbag operation. ARB bars are all certified, so don't worry about that.
If they try to claim that the rear speakers won't work because of the bullbar, I can assure you (as I hope someone else will come along and confirm) that there is a whole other reason, in the D22!