How to manage multiple Bluetooth mics on an iPhone?
There is an order of precedence for Bluetooth devices connected to iPhone. See iPhone Bluetooth Pairing on StackOverflow:


  
    What determines precedence is the order they are connected for that session. The last connected device has the highest precedence and is the one used.
  


Therefore, to ensure the dictation mic is used, connect it after your car connects. If it's already connected when the car connects, you would need to disconnect the dictation mic and reconnect it. (Turning it off and then on again should accomplish this, but without knowing what device it is, it is impossible for me to state this with any certainty.)