There are aftermarket devices that go between the steering wheel and you're new headunit (and which device depends on the head unit.) It intercepts the signals and converts them to what the headunit expects. It about $75.
If you've got On-Star, then your blutooth will part of that system, and you'll need a headunit that can interface with it. Otherwise, you will need a headunit that has blutooth in order to maintain that functionality.
I suggest you go to a local independent car audio shop. They're going to be able to hook you up and you'll be able to get it working for you.