If you wanted to retrofit your vehicle, why can't you just mount a nexus 7 into the dash? I know for a fact that others have done it. I have seen it on reddit multiple times. Will that not work once L-release is on the nexus 7?
I've seen siri fail over and over on the same task, the user becomes frustrated and just does it manually. Siri hasn't matured past the point of an amusing gimmick. Google Now /voice search works perfectly for me, does more than siri, does it better and it's available for iOS. I would recommend to anyone who has given up on the digital assistant concept.