Just out of curiosity, if you have the indash nav built in, can I ask why you're trying to get this to work? I understand that, it should just work as it is, but what is your specific reason?
Having nav doesn't really have any bearing on being able to receive text messages. As far as SYNC is concerned, text messages never happened - I hear a beep from my phone but nothing in the car responds in any way.
I almost never use texting in the way a teenage girl does but I have set up servers and other systems in our office to text me in the case of problems - kind of like pagers in the old days. Some messages require immediate attention while others can wait or are just informational. It would be very helpful to have those messages read to me when they arrive so I can determine if I have to turn around and head back to the office right away.
I certainly wouldn't do anything as foolish as trying to read a text message while driving and I would rather not pull over to read a message that might be just a minor problem.