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+1. The trailer connector is "smart". I would amend that to actually be "not stupid", and there is a fair amount of subjectiveness between "smart" and "not stupid".
But what CR172 said is true. The truck "knows" when there is a trailer attached, and does not send power back there unless a trailer is connected.
I attached my auxiliary reverse lights to the 7-pin connector and they come on each time I put my 2017 into reverse.
Perhaps he needs to attach a light to it or he needs to install a fuse or relay...but it should work as OP wants it to work.