I have 2 reversing lights on my Scania R620 unit, so if you have one on your unit, then adding a reversing light to your trailer would not make much difference to the MFU. if I decide to do this mod I will remove one of the reversing lights from the unit and stick it in the tamiya trailer I am building at the moment
there is a video of a Bruder trailer is here http://www.youtube.com/watch?v=f4QLgxsMEAI and it shows the reversing lights on the unit, but no reversing light on the trailer,