I've got a 3DR Hex equipped with an APM2.6 and a Taranis X8R receiver.
I took it apart a couple of months ago to go over things, check connections, and tidy up the wiring. It stayed apart because the Jan/Feb weather was horrible.. (-15 to -30C with 20-70km winds just about every day).
I just put it back together and there's one connector coming off the power distribution board that I can't recall where it goes.. (It's not mentioned in the assembly instructions.
(As a point of interest, I powered it up my hex with the connector just hanging loose and everything seems to work.. ESC's power up, as well as the APM and the Receiver)..