The "5 volt issue" does exist. Whatever confusion has previously been caused (you cannot generate more than 5V from the Arduino with its 5V Vcc), you do need to scale the voltages so that no more than 5V is applied to either an analog or digital input. If the resistances in the switch are more than a few hundred ohms, you need an op-amp to perform the voltage division with resistances at least 20 times those in the switch, to feed the Arduino.
Okay, so I have to be sure the cruise control switch sends no more than 5 volts? This can be done with a resistor - 1k or greater? The output voltage is not important as the arduino is used to activate 2 x relays and 5v should be enough of a trigger the relays. The Arduino unit does not interact with the transmission ECU - just the up/down mirco switches. The arduino unit is not used in the cruise control circuit, it is isolated from the cruise control ECU. The signals from the cruise control switch can only go one of two ways at any one time; to the Cruise Control ECU, or, the Arduino.
The 9v, and other signals, from the cruise control switch are not sent to the Arduino unit - the changeover relay switches when the shifter is pushed into triptronic mode; push shifter into triptronic - changeover relay switches and signals from cruise control switch ae sent to the Arduino through a circuit that contains a 1kohm+ resistor - Arduino reads the signals and switches relays.
The audio controls is probably a waste of time, but I dont mind having a shot at it. To me, this would be the same as the cruise control switch - just be sure I send no more than 5v to the Ardiuno input. If the Arduino only outputs 5v or less thats even better - the HU/Stereo accepts a 5v input for steering wheel audio controls. I just program the Arduino to send out signals with greater deviation to the HU/Stereo.
I've jumped the gun anyway and purchased an Arduino 2560 and a few other parts.