Since you already have an OBD bluetooth adapter that plugs into your car why not just keep using that and find another bluetooth module so the Arduino can talk to it?
Try this: open up (or look inside) your OBD bluetooth adapter and make note of the board that is soldered onto that adapter's circuit board; that's the bluetooth module. Then go on eBay and look for cheap bluetooth modules. I think they all use those same, cheap, bluetooth modules for communication.