Sorry, should have said that I've checked the fuse and also, when I press the hooter, I can hear the servo clicking under the dash somewhere, so there is a signal getting through, but I think it might be the actual horn (siren) part that's not working.
To test the fuse, I swapped pin40 25A with the ESP 25A fuse (which does a very clear "BRAKE" error if it's missing / not working). Same result (clicking under the dash, no "big noise" up front).
Apparently it's possible to get to the horn by removing the front grills and checking the voltage, but I don't know which side I'm going for, how much space I'll need, how messy this is gonna get, etc... I was hoping for a guide of some sort as to how I can get to the horn to test it.
"Hi" and "Lo" Horns are located on each corner of the front bumper. Easiest way is to pop the respective headlights out.
I would suggest popping out the n/s (passenger) headlight and there should be a "bumper" connector - you may want to check this if you can see it - usually just clipped on behind the bumper skin. Also has wires for fog lights are in this loom - see if these work may also help diagnosis.
The only picture I have is of the washer bottle when the front bumper skin was off and in the foreground you can see the n/s horn unit with no connection - as wiring loom on the removed bumper skin.
(Took this when fitting headlight washers and new GmBH bumpers.)
My o/s Horn failed - repalced it with a Cabriolet horn for the time off Ebay - but note the connector is in a different orientation so o/s horn actually faces outwards to the o/s and not forward.
Errrrrrh confused ???? - post tells where horns are, and there are plugs on each horn unit to test, and another plug on the n/s that may have detached - easy to check by checking front fogs, although could be a broken ground wire common to both horns?
It looks like the fog ground and horn ground are common wires - perhaps this has broken?? - they use the same "4K" labelled connector.
I have once got access to the o/s horn connector through the fog grill - but it was a major pain getting in and around to unplug the horn connector to test - headlight access is a lot easier.
Maybe this will help a little more - circuit diagram showing horn relay J4 and various in-line connectors,