You don't state what year your car is ??
Possibly a 2010 ?
I don't know if it is wired same as 2005-2009...possibly so.
2005- 2009 cars work this way:
All six bulbs are two filament.
Innermost two bulbs (towards center of car), though each bulb is two filament, only one filament is used. There is no brake light or turn signal light from these innermost bulbs.
Only the running light (dim filament) is used. Don't know why Ford did this, but true. So on these innermost sockets there are just two wires each, one for power to running light and one from ground.
You might check to see that all bulbs are correct, all bulbs working in some fashion, not an incorrect bulb used for any one socket, and no one has changed wiring for any one socket. And sockets, all 6, are put into the proper places within the tailamp/brake lamp assemblies.