I would be surprised if they are unmodified 5V/0V USB. Unmodified 5M+ USB runs are pretty dodgy for reliability, and I doubt BMW would use such a system.

I'd wager there is a line driver, probably 12V and perhaps USB 1.1 to keep EMI emissions down. USB2 is like 480Mbps so 450HMz+ via unscreened cable would be a problem.

Is the 18 pin socket the same for E60/E90? I seem to recall white & yellow are used for USB in those cars, but the memory is a bit foggy.