As stated there should be plenty of room behind the front grille area to mount a moderate sized air horn (if you're serious and not just having a laugh).
Only advice I could dispense after messing around with BMW electronics the past few years is to never risk overloading a particular circuit on these cars. If you plan to replace stock equipment with something that has significantly more draw, use a relay and a direct 12V source. The power distribution in this car is finicky - you'll likely not run into an issue if you ignored this advice - but I promise you'll be out quite a few AUD on the odd chance something goes.
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