I have. As the road I live off is in the process of being paved, my truck is crusted underneath with road base. Otherwise, I'd tell you the exact wire.
I'm not sure what benefit finding a reverse wire close to the HU would be, as the camera is so far away. There's nothing inside the cab, anyway, as the harness from the rear is along the driver's side frame rail. Usually, aftermarket cameras have a short tail for the reverse wire and are intended to pick up that circuit at the tail light.
It's easy enough to find the correct wire. Pull one of your tail lights and note which wire goes to the correct bulb. If you're still uncertain, turn the key to ON without starting it and place the transmission in Reverse. Probe the wire with a 12V test light and it will illuminate. Place the truck in Park (or in a forward gear with a manual transmission) and turn the key to OFF. The light will not illuminate when probed.
Usually, aftermarket cameras have a short tail for the reverse wire and are intended to pick up that circuit at the tail light.