Chev98

I got a 2013 gmc sierra 1500 5.3l and having issues with my reverse lights. They seem to work when they want... unless I hit the top of the exterior fuse block closer to the cab. So I'm assuming it's a loose connection but I'm not to sure... hoping it's nothing worse. Would anyone happen to know which wire it would be or at least plug I'm looking for? Or any other ideals that could be causing this problem? I'm not really wanting to bring it in if it's going to be something simple to fix.
Thanks for any help given