It's odd that the top one won't move to AC because that's the "default" position if the actuator was broken, as the top door just lays flat on top of the heater core due to gravity. The bottom door's "default" is in the heat position (go figure).
In any case, you're gonna need a new actuator (which includes the top arm) and of course the gray arm. The link can be reused. I just talked to Max at Five Star Ford yesterday and both parts are still available.
As for the root cause, I think the basic problem with the actuator is that the link between the arms is slightly shorter than the distance between the center of each arm's pivot. I don't know if that's by design or what, but it's obviously a problem if you know anything about geometry. I used to make reinforced actuators which eliminated the top arms from breaking, but all it did was move the stress to the lower arm,and now it breaks instead. I'd be curious to see if your stepdad's car has a modified actuator because the gray arm breaking is pretty rare.
