If you've paired the phone with the car, and if bluetooth is enabled on the phone and in-car, then they should pair automatically. However, I use to have problems with my Moto Droid X and my in-car Moto Bluetooth handsfree speaker system, so your mileage may vary.
I currently have a Razr and I dock it when I get into the car. I use Tasker to sense that it's in the dock and turn bluetooth on an enable GPS. Within about a minute, my phone pairs with the car and I'm ready to go.
Hopefully you shouldn't have any problems...