2014 GMC Sierra

I am trying to help my brother who has a 2014 GMC Sierra with the backup lights staying on. I purchased a used fuse block at a salvage yard with the hope of saving him a few dollars but the problem persists. Maybe the problem was not the fuse box but something else, or maybe I have another faulty fuse box. My brother said that someone at the dealership said there is a junction box near the rear wheel that should be checked for ground. I am writing to inquire any any folks out there have experience with this problem and suggestions regarding what to check next. Many thanks