I have the same issue. I was so mad yesterday, after calling Apple Care and getting someone that didn't even understand what I was talking about, that I went and returned my iPhone. Got a Samsung Galaxy S2 and HATED it. Went right back and got my little iPhone back! Figured I'd have to either disable my bluetooth in my vehicle or not use Siri or Speak to Text in my vehicle. Glad to see others having the same issue.
I have a 2011 GMC Terrain with a very high tech bluetooth system. Now that I realize Siri is meant to work through bluetooth, it would be a nice feature if it REALLY worked. Too much interference, the emails, texts, commands, etc. turn out to be a joke. If I turn bluetooth off, then Siri and Speak to Text works great. But, I want my bluetooth for a safety feature.
I'd like to see an option of disabling Siri and Speak to Text going to bluetooth. HELP, APPLE!!!