I have a 2012 VW Beetle, and I had an issue placing a call with Siri on the watch. I used Siri on my watch to initiate the call instead of the car's built-in voice controls. The first ring tone went through the watch's speaker, then it cut over to the car's audio. When the person I was dialing answered, though, it cut back over to the watch. I would have preferred to keep the call through the car's audio, but there wasn't an obvious way to select the source on the Apple Watch like there is on the iPhone. However, I've found that my VW audio system doesn't play well with anything after iOS 8, so it may be just another issue unique to the VW.