Hi The car is E300TD 1998. Had this problem for a while. My horn quite often does not work. "Hummering" it etc does not work. But what always resolves it is switching ignition off then on. So the actual horns are OK, so is clockwork spring as it is hard to imagine how switching ignition off and then on again would fix it even temprarily. I did replace a relay in the fuse box. What else could it be? I have suspicion there is a circuit to switch off horn when car is being started - may be that circuit activates when it should not? The other problem I noticed is that sometimes the message indicating bulb failure appears but actual bulbs are OK. The other problem I had in the past but that resolved itself was with air heater in winter. It was just non responsive sometimes to tempreture changes on the controls. I had that for two years and then it fixed itself. No problem now but may be Horn is a related problem somehow.