If not a circuit fault (short to 12V) it can be the GEM or
BCM (which does the switching of the lights with solid state devices (AKA transistors, MOSFETS)).
There's a test procedure in the Ford manual for the exact symptom of one parking light always on.
To tell if it's the GEM or the circuit you disconnect a particular connector from the GEM and if it's still on then it's the circuit, otherwise it' the GEM.