Yes, the power you are seeing is from the other car.

What you can do is disconnect that 3 pin connector and then see if there is any difference.

Modern Fords employ, what is known as a "Smart charge" system, basically the regulation etc is done by the ECU. (This is what that plug is for)

With this disconnected the alternator's operation reverts, to that of a conventional system, that way you can establish if it's faulty all together or just one part of it.

Martin