I recently retrofitted rear fog lights on my 09 MCS. I found myself rather annoyed with the lack of information to make both rear fog lights work, instead of the default coded (left fog light).

I only found one thread which sort of helped -> R56 Rear fog light does not work completely. What I found is the important bits mentioned by (@miniwpbear and @guason_jin) to change were not present on my FRM (FRM2_R56.C0C).

I compared both FRM3_R56.C37 and FRM2_R56.C0C and here is what I discovered!
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FRM2_R56_C08 - FRM3_R56.C33

Effective Voltage Coding Data 1
NSL_EINSTEITIG_VERBAUT - Single (Left) Rear Fog Light
nich_aktiv - not enabled [Data 00]
rechte_seite - right side [Data 01]
linke_Seite - left side [Data 02]

FRM3_R56.C35 - FRM3_R56.C37

Effective Voltage Coding Data 1
NSL_EINSEITIG_VERBAUT_C34 - Single (Left) Rear Fog Light
beide_aktiv - both enabled [Data 00]
rechte_seite - right side [Data 01]
linke_Seite - left side [Data 02]
nich_aktiv - not enabled [Data 03]

I found the earlier FRM's [[i]FRM2_R56_C08 thru FRM3_R56.C33] does not have the beide_aktive (both enabled). However, that is not true! I found that nich_aktiv - not enabled is actually beide_aktiv - both enabled. If you look at the data it makes sense, both are 00. It's just incorrectly named, which has caused a bit of confusion for the folks that wanted to enable both rear fog lights.

I added a FSW/FSW Parameter of nich_aktiv - not enabled [Data 03] in the module for my FRM, and I confirmed that is what disables the rear fog lights.

Hopefully this helps!

 

That's some good information on the coding for the rear fog lights........