I've installed a couple of them in 2015 Silverados. About 10-12" above the box floor on the rear left side. Go under the truck and you can look up between the inner box side and the outer box side. I put them about 10" ahead of the tailgate opening. Lay a towel or tarp down to catch the filings from your hole saw or they will make a mess. I also painted the edge after cutting the hole. I bought the ones that just plug in between the 7 pin by the bumper and the wire that feeds it.
B.O.
Former Ram/Cummins owner
2015 Silverado 3500 D/A DRW
Yup I'm a fanboy!
2016 Cedar Creek 36CKTS