One new data point tonight: the multimeter shows 2.5ish volts on the CAN pins on the known good cable connected to the OBD connector, which suggests that the pin numbers listed on that PDF are incorrect. I can't find anything more authoritative for the 2012 model year, and I'm not really sanguine on the prospects for poking around with the multimeter trying to find the the wires that way, so for now, I'm going to go with plan B and find an in-car IR extender just to get something working. I'll keep my eyes peeled for some sort of official documentation on where to get the CAN-B connections, and see if I can find anything on the main CAN bus.