If your car has a built-in speakerphone, or if you have a Bluetooth accessory that has a microphone and speakers, you can choose which one listens to for commands. When you click and hold the iPhone’s Home button, you’ll see a speaker icon to the right of the main button. Tap it, and you’ll get a menu of all the various sound inputs your iPhone can use; select the one you want, and will listen using that microphone and, if possible, give its feedback via those speakers.