Remove the phone from the device list in the vehicle. Remove the vehicle from the device list on the phone. Do a reset on the phone, holding the sleep/wake and home buttons together until you see the Apple logo and then release. After the phone restarts, turn Bluetooth on for the phone. Then go into the vehicle and follow the manufacturer's instructions for putting the vehicle into pairing mode. See if you can pair the two devices.
On occasion, devices will not always connect automatically. If after the devices pair this happens, just go into the Bluetooth setting on the phone, locate the listing for the car and tap on it. It should connect then. This will prevent the need to try and pair the two devices again.
You can also see if this support document helps you any. http://support.apple.com/kb/TS3581