Most people when driving are not concentrating, with most actions being done in their sub-conscious.
Our brains are not designed for motoring, we have developed technologically far faster than our basic instincts have developed. When we look at moving objects, we are poor at judging speed. One continuous look is not that good. We have to look twice, then our brain can compute the distance trravelled against time, and understand how long before the object arrives. Size is also important.
An emerging driver will see a small car, and sub-consciously equate it with slow. A larger car, even travelling slower, will be waited for. Then as the size increases to a van or truck, slow becomes the expectation again. This is also why people pull out in front of motorcycles, as the brain categorises them with bicycles. In my experience, a motorcycle headlamp makes no difference, but removes the ability to use the headlamp flash as a signal/warning. A white fairing on a motorcycle keeps cars away. Their first view alerts them with the question, "is that a policeman?", and then of course, they 'see' it in their conscious thoughts. Had a small motorcycle with a white half-fairing, never asn issue with cars pulling out. Later transferred the fairing to a medium motorcycle, still good. Later bike, full, large black fairing, everyone out in front.
Sadly, with a small car, they'll pull out. Colour makes a difference too. Strangely, blue cars seem to be invisible, even medium sized ones.
When fitting the horn, the two wires at the standard horn connector can be taken to the switching coil of the relay. An eBay search should find a connector to make it neat, so no cutting/splicing. That's how I did the Panda, now with two Fiamms, one each side hanging behind the bumper under each headlamp. I think the 500 has less space.
Mount it away from any ECU. The electromagnetic field upsets the ECU. Fitted a pair of Fiamms on a Seicento, only space was under left wheelarch, behind liner. Sadly, close to the engine ECU. Press horn, eng management light comes on. No apparent issues, and went off at each restart, but risk too high, so disconnected.
