Reverse lights issue HELP!
 

Im having an issue with my reverse lights on my 2013 GMC Sierra 1500 5.3L 6L80e transmission, and I don't have any lights. I've check the obvious fuse and bulbs and both are good. The fuse block and the junction block are also new within a few months and transmission was rebuilt. And brought it into my local chev dealership and about all they can tell me is a wire is loose/ broke.
What I'm finding is if I have a constant power (wire from + on the battery) to the fuse for the vehicle back up lights they will come on. So I guess what I'm needing help with if anyone had this issue before and if what was the problem? Or if anyone would have an ideal of where to look? Wire to look for between an area, or anything that could help pin point my problem.
Thanks for any input/ help on this