Mapquest works with OnStar as well. For Google Maps, you need your OnStar User ID, but for Mapquest, you need your Account # and Zip Code.
To my knowledge, that's correct. OnStar Turn-by-Turn does not do traffic, but the Nav system does. So, if there's heavy traffic, I doubt Turn-by-Turn would match your Garmin's (or Nav's) route.The Onstar turn by turn does not do traffic, correct? The in car nav system does though, IIRC.