The towing vehicle is a 2008 ML320 CDI with a factory installed towing package. There have been no problems with trailer lights with our past trailers. In the past two months we have bought two new small trailers. When the first trailer lights were first connected, by the trailer technician, the trailer lights would only flash. I had the vehicle running when the connection was made. I shut the 320 off and restarted it, with the lights connected. The lights then worked and still do.

Two days ago we picked up the second newly built and delivered trailer. It is an enclosed 5' x 8' with an interior light and LED brake, turn signal, fender clearance lights and tail lights. When we hooked up the light harness none of the lights would function except for a couple of tail lights at a low intensity. I went through the same routine of shutting off the engine, re-connecting the light and restarting the 320. No combination of engine off or on, lights off or on, made a difference. The dimly lit bulbs are part of a 5 bulb light bar on each side of the rear of the trailer. These dim lights come on when the lights are connected whether the 320 is running or not and whether the lights are on or not. None of the other light functions work; brake, hazard, turn signal or tail lights.

The technician tested the wiring on both the vehicle and trailer and all checked out as working. He tried a different adapter plug and wiring from this adapter to the trailer and still no lights. The trailer wiring was hooked up to the dealers' portable tester and all the trailer lights functioned as they should. The technician said he had not encountered this issue before, and he has worked at this trailer dealer many years, and said he was not familiar with Mercedes-Benz vehicles. He was the same tech that checked out our first trailer.

We towed the covered trailer home and as a test I hooked up our other trailer. At first the trailer lights flashed then began working as they should. This trailer is also equipped with LED lights although not as many as the new covered trailer. I re-hooked the newest trailer and nothing worked.

Any advice or sharing of similar experience is much appreciated.