Solved: Aftermarket trailer wiring - shop won't do brake input?

[RhythmMachine]

Called the local hitch shop to see how much it would cost to get a hitch and wiring in for an X5 I'm considering buying, and they said that they don't wire in controllers for BMW because of the coding that needs to be done. I'm pretty sure I still have ISTA and WinKFP on an old laptop, so should be able to do that myself, but the wiring they'll run up under the dash is for power and brake output. They said they don't do the ground (which I should be able to figure out) or the brake input on BMWs.

Any idea if wiring up the brake input line for the brake controller is relatively straight-forward as a DIY?