From the complete install instruction I received with my hitch:

14. Carefully route single blue/brown wire from
trailer wiring harness along existing
harnesses to area behind right tail light
assembly. Secure all harnesses with wire-ties
(3) & (4) from installation kit.
15. Locate and remove Connector X318 (1) from
Tail Light Assembly.
16. With connector X318 (1) removed from tail
light assembly, use BMW approved tap
connector (3) p/n 61 13 8 364 566 [included
in kit] to connect blue/brown wire from trailer
wiring harness (4) to the blue/brown wire (pin
3) in the connector. Make certain single wire
from trailer wiring harness does not protrude
from tap connector.
17. Reconnect connector X318 to tail light
assembly.

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Sent via Telegraph Wires using my Morse Key.
2021 X5 xDrive40i
2015 X5 xDrive35i MSport
(Traded) 2013 X5 xDrive35i