The oem trailer wiring options don’t power trailer brakes either, so finding a brake signal for the brake controller is always an issue. On my mk6, which I think has the same bcm as an nms, I was able to get the signal from the brake output on my trailer module, which isn’t normally used when coded to NAR. I just ran a wire from the module to the controller when I installed the oem trailer wiring.
On my MK7, this didn’t work. The module cuts the brake signal output on the trailer module when recoded, so I had to get the signal elsewhere. At first I used the brake signal wire from the bcm to my trailer module. It worked, but did some weird things like coming on when the car was off that made me wonder if it was working right.
I did a little reading on a Tiguan forum, and guys said they had no problem using the output from the bcm to the third brake light. It’s usually an led, so the car doesn’t check it the same way, so the brake controller doesn’t throw bulb lights. I tapped this wire on my Golf and the brake controller has worked properly ever since. I can make the car tell me to check the third brake light if I keep pulling the manual override lever with NO trailer attached, but that’s the only time it’s come on, and not something I ever really do.
My bought back JSW had the same BCM part number as my current 15 NMS, so I assume it’ll work similary. If someone with Erwin can figure out what output is from the bcm to the third brake light, I think it’ll solve the op’s problem, and be easier than tracing it from the back to the front, which some of the Tiguan guys did.