One problem related to horn assembly has been reported for the 2007 Mercedes Benz ML350. The most recently reported issues are listed below.
While driving down the highway I smelled what smelled like wires burning then my windshield wipers turned on by itself and wiped at a very fast pace, then the horn started blowing (on its own) very loudly, I pulled over at a tire shop and I noticed smoke coming from the rear passenger door panel. The fire department was called and they disconnected the battery. When I spoke to a mechanic I was told that there was a recall on the wiring harness and that was more than likely the problem.