You can fix your wiring by adding a couple of high-current diodes to the trailer wiring harness, need to prevent the blinker voltage feeding backwards into the brake light and vice verse.
I still find it weird the way Nissan wired the brake kill circuit slaved off the brake lights themselves, but I guess it makes sense in a weird way.
BTW I told you it was the kill switch, just didn't think about it from that end of the car.
