I saw a guy on the Focus RS forum was trying to add a rear fog light his RS. The Euro RS comes with a rear fog light you turn on from the light switch module. Based on his findings, the US vehicles will respond to the light switch signal by illuminating the icon on the cluster but the BCM does not output the signal to turn on the rear fog light. The BCM firmware can only be updated by a dealer and no dealer will install the Euro BCM firmware. The Focus RS guy gave up an added a separate switch. I looked for another solution.
I bought a light switch from eBay UK. I confirmed the rear fog light illuminates on the cluster and the BCM does not output a signal. I tried in vain to find a signal from the light switch module I could use to turn on the fog light. The switch is only sending out LIN Bus packets, there is no physical switching of signals/power.
Next, I focused on the power that was controlling the LED on the cluster and found the circuitry for the rear fog LED on the cluster circuit board (I used my spare cluster from a recent cluster upgrade). The LED does not turn on/off with a simple on/off voltage and the power is pulse width modulated (PWM). After a lot of testing and some circuit design, I found a circuit that will turn on a rear fog light by pressing the button on the light switch.
I built a circuit that reads the voltage on the cluster LED and turns it into a signal I can use to drive a relay. I know this is probably too complicated for most, but it is a functional solution. See the attached sketch of the circuit diagram. Half of the circuit is home-made and the other half is a relay driver that I bought on Amazon. I soldered a wire on the cluster circuit board for the LED signal and connected it to the circuit (I briefly panicked when the new, upgraded cluster PCB was completely different from the old, but eventually found the same circuit components). I used power from the front fog lights to operate the circuit and the rear fog lamp so nothing is powered if the front fog lights are not on. The circuit is under the dash cover behind the IPC.
Next, I had to modify the socket for the rear marker light. I bought a used wiring harness with all of the rear bulbs and sockets. I carved out the marker light socket to match the keys on the larger socket. It fits perfectly and the 2-element socket locks in place like OEM. The “bright” element is the fog light. The “dim” element is the marker light. I am still experimenting with different bulbs. The lens is not designed for projecting light rearward. A 7443 LED seems to work the best for providing a bright red fog light.
Maybe someone (Chong?) will design a circuit using an Arduino that will read the LIN Bus and close a relay when the fog lamp switch is pressed. This is beyond my skill set.