If your car has a GPS receiver in it, doesn't it also have its own satnav software?

Does your car's GPS have a data port on it? Does it have BlueTooth? There are replacement BT stacks (jailbeak required) for the iPhone to allow connection to an external BT GPS. Though I've also read that the iDevices now seem to work with BT GPS receivers, so perhaps the replacement stack/app isn't required any longer.