Isn't that the crimp from the brake magnet to the trailer wiring?
I had to track down an issue last year with a magnet and cut all 4 sets of these wires and reconnected one at a time to determine which was bad. At any rate, user on a Ford forum I had posted my issue at recommended these for reconnection:
3M Scotchlok Connectors
He also recommended a dab of petroleum jelly in each one to guard against corrosion.
As an FYI, my crimps were always visible - not tucked into the backer or anything. Curious what others say about that.