I would check the trailer wire harness where it attaches to the frame behind the rear bumper (driver side). It rubs on the frame and has been known to cause tail light issues.

 

I have a 7 pin trailer plug, i assume aftermarket U-Haul install. It has a jumper taped into the right side Backup green wire. I unplugged that first. No joy. are you saying that there are other wires in the backup circuit at the trailer harness?

 

Open the back door, look between the bumper and chassis for where the wiring harness goes to the license plate light and trailer hitch. Common problem is the wiring harness rubs on the chassis because it is pretty tight (need to be longer) right there and causes a short.

It killed my battery several times as it was grounding a hot wire. Just like Tainter said, look there.

You can tell if you have an aftermarket trailer hitch/wiring real quick, factory trailer package has a cut out in the rear bumper so the hitch receiver tucks up higher in the bumper. Like this....

 

anyone with factory hitch should just push up on cable till the clip unhooks or breaks,, job done
if it has not worn thru already. tape each wire if worn

 

OK, Update, Got home from work and crawled under rear bumper for a closer look. Well, it's a fact, I have an aftermarket hitch install, evidenced by the big orange U-Haul logo on the hitch.Surprise!
I had just enough time to clip a couple zip ties and unwrap some tape before I realized the harness (that did appear to have some chaffing) was out of reach without removing the hitch, oh goodie! I,ll start on that this morning, then I will completely expose and inspect all wires, repair as needed, rewrap, reinstall, and move on to the next item...could I be that lucky?? We shall see. I still believe this may be a failed exercise because I already found where they (U-Haul) tapped into the Backup lights and disabled it, and the fuse still blew. My hope is that there is also a factory Backup wire in the harness and they bungled the job from the start, and nobody ever noticed.

 

Here we go, Update #2,
Backup Fuse pulled, Bumper and hitch removed, Taillights removed.
All trailer wires unwrapped and exposed and inspected for short, none found.
The only wire connected to the hitch plug (center conductor) was going to a tap on the light green wire (backup bulb) in right taillight, no short in it, but unplugged anyway.
With vehicle off and in Park, Pulled both bulbs, tested for continuity to ground from light green wire and got it. Unless i'm missing something, the light green wire is shorted to ground between the backup switch and the bulb socket
Before you ask, this vehicle is still in close to new condition, no accidents, damage, modifications (except for hitch) and from the looks of the bottom, hasn't even been off road.
Soooo, is my next item to check the switch??

 

Finally, a conclusion...well almost.
Crawled under tranny after putting truck up on ramps. found the shift position switch, removed heat shield to gain access to plug.
Unclipped and removed plug and got a face full of deteriorated wire loom. Oh my, reached up and noticed the harness above the tranny was dried out and partially missing, ran my hand up and over as far as I could, went over to the other side and found unshielded wires going through a metal clip on top right of transmission. did some preliminary poke, wiggle, shaking of wire bundle trying to figure out how to gain better access to identify and repair any short and rewrap (very tight). Just for the hell of it, I went back to the reverse light socket and tested for continuity to ground and IT WAS GONE!!! I found it, now I have to hire a mechanic with a lift to get under it and repair. Just can't get enough access lying on my back, looks like the exhaust may also have to be removed.
In closing out this thread, my advice to anyone who lives in a dry, hot climate (like Phoenix) inspect the condition of wire harness over transmission as preventative maintenance, there are lots of small wires in this bundle that could cause much larger problems than backup light failure.
Oh, and thanks to everyone who shared their knowledge with me!!

 

i wonder if you took any pictures, where the problem is. look like i have same problem, except no blown fuses, just plain dead back up lights

 

We pointed out to avoid digging up OLD Threads by tacking you question on the end of one.