I had the same problem so I was watching this thread for tips. It took 3 or maybe even 4 hours to track down my bad short.
I actually found 4 spots where the rear harness was chafed bare and rubbing the frame:
The first was where the flat 4 wire harness went around the parking brake. Every time the brake was applied it moved the wires a bit.
The second was at a support by where the exhaust hanger was.
The third was where longhorn man called it- right where the green wire to the backup lights goes up through the rear of the bed. Both sides where actually chafed.
And after fixing all those the fuse still blew instantly.
But the fourth was the culprit. The friggin license plate light wire had a long black wire pigtailed to it that went around by the bumper bracket, and I had wrapped it around the bumper bracket when I put the bumper back on after paint, not knowing where it went. And that was of course the one causing all the problems.
But during all that I also found out that the backup lights run off that same circuit so maybe try disconnecting those by the neutral safety switch under the dash?
