All, I have discovered another workaround.
4.6 users, try this:
1. Allow your phone to connect via Bluetooth.
2. If you plan to use AppLink, connect your iPhone via USB as well.
3. Press the hard button on the phone to activate Siri.
4. Wait for Siri to to give you the "Some things you can ask me" screen, then press the little Bluetooth icon in the lower right corner.
6. If you are connected via USB, select "Dock Connector." If not, select "iPhone." Note that iPhone or Dock Connector may already be selected. This, I believe, is the cause of the "bug that isn't a bug."
7. You are now using the iPhone's microphone and speaker for Siri, and it will work as expected until your phone switches back to Bluetooth. This can happen for various reasons, e.g., if you get a phone call, switch your streaming source to Bluetooth, or one of your apps attempts to play audio as a Buetooth phone call. Then you will need to perform the process again the next time you attempt to use Siri.
@FordIVTeam, could you all please consider an enhancement request that Siri work smoothly when the iPhone is connected via Bluetooth? Bonus points for an option to let us use the steering wheel button to talk to Siri instead of Samantha. We really want this because it's so much safer to have Siri available to respond to texts, find apps, addresses, songs, etc. I'm sure this has been mentioned dozens of times, but a $150 radio from AutoZone can do this with no problem. It's very upsetting that our integrated system can't do it correctly.