The LED lights on the trailer do not pull enough current so the ML doesn't know that a trailer is even connected, therefore never turning the respective lights on. There are available in the after market an LED inline trailer module to simulate an incandescent load on the circuit. (
U-Haul: Moving supplies: LED Light Trailer Module) Thats just one example. Almost all european vehicles require some electrical load to "turn-on" the trailer lighting. I know that a lot of American and Asian manufactures are starting to do this as well for the trailer sway programming in the ESP. If the lighting and trailer stability program was just set to on all the time, the ESP system would just assume a trailer is always connected and your traction control would not operate correctly.
Hope that helps!