On an e70 there is a motor in a box then cables to the hubs ,it used to cost £1000 for the genuine part from what I remember , they can be got cheaper now though., it likely won't be a cheap fix which is why they havent sorted it .The motors wear out , you cant buy just a motor genuine though I have seen chinese ones and its not serviceable .as far as. bmw are concerned.
Ah, ths world of BMW has many ways to empty one's wallet. I just Googled and ended up on YouTube, which confirmed what you suggest. I'll give that car a swerve i think.
The early ones used to strip the gears and were mega-pricy. It’s one of the reasons that BMW switched to placing the actuators on the rear disc. The other being of course that not fitting a drum brake for the parking brake saved them money.