I was able to obtain the schematic for LCM plug X12 to be used for the brake signal pick-off. With this descrpition and the attached diagram it should be easy for any installer to find the correct connection point.
"A connection pin for retrofitting (electric brake) is provided on
plug X393S. If an electrical brake is desired for the trailer, it
can be retrofitted. For the latest list of recommended brake
controllers contact BMW Service Hotline. The signal pick-off for
brake (the input to the brake controller) may only take place on
plug X12, Pin 10. This plug, X12, is to the Light Control Module
(LCM), the wire color is black and yellow. The male pin (BMW
P/N 61 13 1 376 195) should be used at the mating plug of
X393S. Be sure to observe this information."