Is your car connected to your iPhone using Bluetooth? In other words, if you were to make an iPhone telephone call, would you use the car's microphone, or speak directly into the iPhone? Use the same method for Siri that you do for an iPhone telephone call.
Also, hopefully you are not relying on "Hey Siri" to awaken Siri, as that requires your iPhone to be plugged in using an iPhone 6.
See "Talk to Siri in your car" here:
Use Siri on your iPhone, iPad, or iPod touch - Apple Support