See this support document to be sure the vehicle supports the Phone Book http://support.apple.com/kb/HT3647. I believe it does, so you need to put both devices in pairing mode and connect the two. Once you do that, the vehicle will take the phone book entries, or you may have to do something in the vehicle for it to pick up the entries, but you don't have to do anything on the phone once they are paired.
Why do you think the sync was not complete? Did you receive an error, or does it have to do with the number of contacts that are in the phone versus on the vehicle?