For Nikon Service information, please see:
Title: How do I get my Nikon product serviced?
There is no need to call in advance for any approval or authorization; simply send the product in for evaluation with no accessories, unless part of the issue, and a photocopy of the receipt. Once logged in, service will send you a confirmation.
Sorry for the problems you have had with your Nikon product; Nikon Service strives to evaluate and repair all items as quickly as possible.