Check out this neat product called Mobile Home. It's a blue tooth button that activates Siri but still let's your iPhone work with your car's blue tooth. Mobile Home™ | Power of Siri® from the Driver's Seat. I have a model Two and would love to figure out how to wire something like this up to the unused button (voice command) spot on the steering wheel.
FriarNurgle don't bother with the Mobile Home. It's more expensive and connection is more flaky vs the Satechi remote which I just picked up recently. Cheaper and more features and no flaky connections.
Yes the Satechi bluetooth multi media remote. Here is a link off Amazon. I bought it 3 weeks ago when it was for $39.99 Satechi BT MediaRemote Bluetooth Multi-Media Remote Control for iPhone, iPad & iMac, MacBook Air, MacBook Pro, MacBook, and Mac Mini (japan import) - Amazon.ca