I would check inside the pigtail shell. The contacts in many of those are very close together and may be touching in the shell.
I had a backup light issue, would blow the fuse on the camper running lights in the truck. It was an intermittent short buried somewhere in the left rear corner of the camper beyond accessibility. It took me a year to find, I eventually had to wire around it.
For debugging this kind of short, the very best tool is a sensitive DC clamp ammeter, will cost you about $80. You can clamp it on any wire and it will tell you how much current is flowing through. That lets you quickly determine which circuit is at fault, and trace through the circuit to localize it.
