I too am puzzled by this entire thread.
If you have a 7-pin connector, pin #5 manages the electronic braking system which your trailer has, so I am not sure what that 4-pin trailer brake controller is for? (must admit I'm not familiar with braking systems on trailers, as my trailer only has mechanical brakes).
Interestingly, my BMW OEM trailer connector on my X5/G05 is a 12-pin connector - see pic below.
The 12-pin adds 5 x larger pins for:
- A fridge ignition feed to power the fridge in your trailer home or trailer while you’re driving.
- A breakaway battery monitor. This allows the voltage of the battery on your breakaway unit to be displayed on the dash of your vehicle.
- ESC, which is Electronic Stability Control, if your trailer home has it.
- a heavy earth return.
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