Depending on if you want to use voice input as part of your car mode or not, you could try the voice app Dragon and then use an NFC Tag to trigger opening that app when you enter your car. Otherwise, I believe by nature car dock mode would be hands free mode since you're not supposed to be messing with your phone while driving. Another option if you never use S-Voice is to go into your list of all applications and Disable S-Voice. I have S-Voice disabled on my phone since I never use it. Doesn't negatively impact anything. Worth a try...
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