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.