There are three separate CAN buses in these vehicles, as shown here and described here. The diagram is a few years old, but I'm told it should still apply to the 2012s. The "FCM (CGW)" box is apparently some sort of gateway that would let me get through to the CAN-B bus from the CAN-C diagnostic bus, and someone on the canhack.org forums claims to have done so, but hasn't described it in detail, so I'll believe it when I see it.