ML280 W164 Horn Sounding randomly

[Rob H]

The horn on my ML280 2008 started sounding randomly whilst driving, and will also not work normally sometimes. I’ve checked the wiring loom as far as possible and cannot find anything obvious. The relevant relay is working fine and is getting +ve when the ignition is on. It is also getting -ve without the horn button being pressed (When the problem occur). I’ve checked the wiring behind the steering wheel airbag and all looks fine but it looks like the horn switch is not just a simple wire that is earthed when selected. I’m guessing that the switch signals the computer and then the computer does the switching for the relay coil. Any thoughts welcome.