@Sean d The attached wiring diagram covers Z3's from 4/99 on wards.
If you are researching the 3 wire cruise control switch matter, you'll see in these diagrams that a three wire hall-effect switch is used to give a signal to the DME (6571.0-3). The issue is that the early M52TU powered Z3's got fitted with a 2 wire switch before the 4/99 mods got through to the factory floor. The 2 wire switch gives the DME a signal that was opposite to what was needed e.g. It said the clutch was in when it was actually out, so the cruise control would not turn on and hold speed.