Hmmm, not sure about this, as far as I was aware Hyundai's media system is not Siri compatible, the system in Hyundai cars is only a Bluetooth one to my knowledge.
At the moment I would suspect that trying to use both BT and Siri it will simply get confused, Siri being voice recognition specific to iOS whilst most car systems use their own voice recognition. Unless Apple license Siri for free to car manufacturers then they will probably avoid it, on the other hand Apple may try and create partnerships with some (I would suspect premium brands) in order for those brands to try and tempt you with their special systems.
Time will tell......
EDIT!
Searching the t'interweb would suggest that Apple and Hyundai have created some form of partnership announced earlier this year, so future models may exhibit Siri compatibility, existing ones will be limited.