[brian_]

As you have already determined, it's hard to imagine that the thicker wires could be anything other than

black -> line pin
white -> neutral pin
green -> ground pin​

Since there are no other pins in a NEMA 5 connector, there only remaining connection which makes sense for the thinner wires is to the line and neutral. Ford might have done this to allow the charger to read voltage at the plug, without voltage drop due to current flow in the thicker wires (which actually supply the charging current). Unfortunately, that's only a reasonable guess, and even if it is right which of the two thinner wires is which is unknown.

If you can open the charger case (without damaging it or causing other issues, such as with warranty coverage), perhaps you can see where the other end of the blue and tan wires go - that might provide a hint, although it's unlikely unless someone has posted internal details of the charger online.

 

[brian_]

It would be best if you can take apart remainings of the old plug to figure it out. Or at least measure the resistance across the thinner wires that are coming out of the old plug.

I was assuming, perhaps incorrectly, that the original plug was gone or destroyed. Certainly if you have it, examine it. If the small wires are just connected to pins, and the plug is relatively intact, you can simply test with an ohmmeter to determine the connections.

Thinner wires could be connected to a thermistor, that measures plug temperature, or/and some sort of digitally interfacing chip, that stores current limit for a particular plug.

I had not considered the possibility of an active device in the plug. It's hard for me to imagine any reason for this other than temperature, but a thermistor seems like a remote possibility.

Some EV chargers have a set of plugs to go onto outlets of different current capacities and voltages, so their plugs must identify themselves to the charger. Given the provided specs this is just a 120V 12A charger, so there is presumably only one fixed plug, and no reason to expect any of this complication. Certainly there is no interface to the receptacle (such as that in a fast charging plug) and no handshaking with the power source.