Check this support document http://support.apple.com/kb/HT3647 about supported Bluetooth profiles. Not all car stereos will support the use of Siri. It has to do with the stereo's capability to connect to the voice control of the phone. You can look at this troubleshooting support document for car stereos http://support.apple.com/kb/TS3581. I am not sure exactly what Bluetooth protocol is necessary for the use of Siri, but you might want to check with Panasonic support about this. You cannot be the first person to ask this question.
Seeing how Siri came out after your vehicle did, it is possible there would be a firmware update to your car stereo. Check with your dealer to check with the vehicle manufacturer for an update.
Message was edited by: ChrisJ4203