Haven't looked any more into this but was thinking of getting a Satechi BT remote to use in the car, will be useful. It will also activate Siri but I wonder whether it will come through the speakers - I doubt it.
I tried the Google app and did a voice search and interestingly this came over the car speakers via blueooth - obviously not the same as Siri and only lets you perform searches without any audio feedback. Thought it was strange this worked and not Siri - may be it's the way it is initiated?!?!?!