The BMW E90 start button is a 10 Pin switch and is not useable at all without an external circuit.
The switch uses 2 identical circuits with contact less magnetic hall effect sensors to detect the button press, the reason for 2 being safety redundancy, to ensure the action carried out is intended.
For illumination you can supply 8v directly, a resistor is inbuilt. If you want to supply direct power from the car (12 to 14v) then you need to include a resistor of the appropriate size, 100 to 200 ohms depending on brightness.
When functioning correctly the switch will provide a ground signal capable of driving 50mA
The Hall effect circuit needs power in order to function, directly powered from the car is ideal (12 to 14v) and either 1 or both circuit can be used in your design, just depends how safe you want it.
This is all experimentary and of course the switch may change without notice from BMW, make any connections to this switch at your own risk!
1 & 2 = Led Illumination
1 = -8v
2 = +8v
Redundant Circuit A
Pin 3 = +12v
Pin 10 = -12v
Pin 4 = -12v Ouput when switch Engaged
Redundant Circuit B
Pin 8 = +12v
Pin 9 = -12v
Pin 6 = -12v Ouput when switch Engaged