[ray_mcavoy]

I can't quite tell from the pic, but does that wire have the same type of black rectangular connector as the heater power wire? If not, it's definitely something someone added. But even if it does have the OE connector it could still be a section of wire someone salvaged out of another GM vehicle and used to power some accessory that has since been removed.

As you already know, that location would have been used to power backup lights if your truck had that option. There might have been some other options that sourced power from that location as well but I'm not aware of any.