Not sure the wiring is that complicated, well you aren't cutting in to wires with scotchlocks, as that would upset the ECU bulb out warnings as well as alot of the canbus stuff. Also the towbar electrics are part of the MOT which they weren't before.
I think the main thing is that the wiring is complicated, which might rule out getting a 3rd party to do the installation.
Think they’re going to speak to BMW and find out how much they charge as a starting point.
There’s even an automatic pop out one that costs about a grand all in, lol.