Brian Gaff (Sofa) explained on 19/04/2021 :
That could be done by a chip called a window comparator, witch is capable of
monitoring the current or resistance depending on how its set up, of each
light, but do people really go to this complexity these days?

Obviously they do, or I would not have one. It is the OEM BMW optional
CAN-BUS part. My car already has/had a full road lighting monitoring
system It cost me around £130 extra for the extra towing module,
compared to the basic towing adaptor sold by the accessory shops. A
single extra wire relays data back to the lighting control module in
the dash. Worth it to know my lighting is fully working on my trailer.