2013 C5 X7, one horn not working

[Y Cymro]

One horn has stopped working on my car. That the other one works suggests it’s the horn unit that’s faulty rather than a fuse (which I’ve tested), relay, or wiring. I’ve tried giving it a tap which made no difference, and the plug-in connections appear sound.

I can see the horns from the top of the engine bay, but replacing the faulty one appears to need removing the under engine cover, which is difficult at this time of year when laying on my driveway.

Is there a better way to swap the horn units?

[Y Cymro]

I’d better replace the faulty horn, so does anyone have advice on how to remove the under-engine tray so I can get to the horn that way, or alternatively remove the NS headlamp unit so I can reach it that way.

[Y Cymro]

Whilst removing the battery to clean terminals, connections and local earths it became apparent that the top horn - the ‘high’ note - is easy to replace with the battery removed, and with no need to remove the under-engine covers.

I couldn’t find an X7 Citroen horn for sale except from a dealer, but a Fiamm from a 2012 on Alfa Romeo or Fiat is the same horn, more plentiful to find on eBay and cheaper, but requires the unplugging of the Alfa electrical connection and reuse of the Citroen plug.