How do I have my Galaxy S3 automatically switch into Driving Mode when I am moving in a car?
How about using NFC to tell the phone that it should switch itself to driving mode when inserted into the cradle?
You can get nfc stickers for few euro/pounds on ebay..., put one to the back of the cradle, setup your phone and off you go :-)
That probably isn't what you're looking for but still, it's nice and elegant solution ;-)