I don't understand, electronically, what the difference is between 0v and open circuit. I thought those were the same thing.

0V means connected to ground; open-circuit means not connected to anything. An input that's open-circuit doesn't necessarily read 0V, especially in the presence of electrical noise.

So some cars switch that input between (connected to +12V) and (connected to ground); that's easy to distinguish. Others switch it between (connected to ground) and (not connected to anything), and in those cars connecting a biggish-value "pull-up" resistor between the input and 12V will result in the grounded case reading as 0V (because the ground connection is better than through the resistor) and the open-circuit case reading as 12V (because the connection through the resistor is better than nothing, so the voltage is "pulled up" to 12V).

Peter