To me, this seems to be a "stalling" tactic, a means to determine how motivated you are to get the job done.
3rd level Brake is tied in with standard Brake, no special switch or wiring except for it's pigtail.
It's LED - it uses it's own dropping resistors and is a self contained low-current draw device that if your other brake lamps burn out or filaments quits - the LED should finish the job - which was the purpose of the 3rd level brake in the first place, to keep people from staring at the wrong end of the end - never paying attention to your hitting your brakes, they didn't - sort of (insurance) thing.
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So the Ford uses 7 wire - 1 Brake, 2 Turn (Amber) - Tied (single) Backup and Tail (running) - 7th being option Brake or Charge (loop alarm if disconnected)
You can get Trailer wiring harness for it to help you sort this out. Don't need the 4-wire conversion - just a means to tap into the rear connector pigtails