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If you're pulling it from the harness that goes into the back of the 7-pin trailer plug (assuming you've got the 7pin plug), then it should be the center pin wire.
There is a wire in the fuse panel beneath the steering column that is 12V+ when the vehicle is shifted into reverse. It is on the lower passenger side of the fuse panel, it is a black wire with pink tracer. Here is a picture of it from where I tapped into it in my truck.
My truck is an 07, I would think it would be the same on the 05-07, possibly the earlier trucks too.
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