Originally Posted by sekazi
Pressing the button puts the car into accessory mode so you get the blowers, radio or anything to do with the MFT along with any lights. Pressing the brake and then the button will start the car so you are using gas not the battery.
I don't think it's a true accessory mode, if it's running the fan, headlights, etc. It's basically turning the ignition "on" without cranking the engine (equivalent to position II with the key).
I think the OP just wants the stereo (what accessory mode should be), and I think you can turn that on separately without powering up all the other stuff (which will make the battery last considerably longer).