How do I force Siri to listen on internal microphone, ignore Bluetooth device?

When connected to a Bluetooth OUTPUT device like a car stereo or portable speaker system, my iPhone 6 / iOS 9.0.2 defaults the audio INPUT to this Bluetooth device as well, even though there is no microphone on this device.

When engaging in a phone call, I can select the preferred audio input device (internal iPhone mic in this case) with nothing more than the annoyance factor of having to select it every time (though arguably it's much less safer to drive when futzing with this setting over and over). Furthermore, in iOS8, there was a small button with Bluetooth logo on the Siri screen which also let me select the input device which Siri was listening on. Again, annoying to have to keep changing it, but at least there was an option.

It seems that in iOS 9, this button has disappeared from the Siri screen, and Siri is now useless to me when connected to my Bluetooth system, because she is ONLY listening for commands on that bluetooth device which as no audio input capabilities.

Please tell me there is a way to force Siri to ALWAYS listen for commands on the internal microphone, or otherwise disable bluetooth connections from hijacking my audio input setting whenever the device pairs/connects.

Thanks!