I come to this thread about five years late but I want to share the results of my recent installation of a pair of Hella air horns. I had previously installed a pair of Hella supertone horns but i thought they did not offer the vintage Euro sound that my '71 Plus 2S should make when sufficiently aroused. The air horns were about $30 which I thought was reasonable. Air horns were originally fitted to my Plus 2 but one of the previous owners saw fit to replace them.
The goal was to find a place to install the compressor and the trumpets such that they did not interfere with other components in the nose of the car, to fit the trumpets so they were pointed downward to keep water and bugs out of the trumpet reeds, To keep the trumpets as close to the compressor as possible so that the plastic tubing from compressor to trumpets was as short as possible to eliminate any delay in sound once the horn button was pressed, and to point the trumpets toward the nose of the car so the sound would be directed forward.
A bracket had been fitted to the left hand fender which served as a mount for the previous horns. I don't know if the bracket is original to the car or fitted by a previous owner. I decided to use that bracket to hold the compressor. Because the bracket did not allow the compressor to sit parallel to the fender I fashioned a separate bracket out of aluminum flat stock. Next, I made another bracket out of the same stock on which to mount the trumpets and mounted the bracket to one of the body/chassis bolts. I then measured the plastic tubing that came with the kit so that the I could keep the tubing between the compressor and trumpets as short as possible without kinking it and shutting off the flow of air.
From the pictures I have attached, you can see the trumpets situated between the vacuum pod for the headlights and the K&N air filter mounted in the nose. You can also see how the trumpets just clear the left hand radiator fan. Since I have replaced my narrow original radiator with a full width aluminum radiator from Coolex I did not have the ability mount the compressor to the radiator bracket.
Finally, I plugged the power wire coming from the horn relay into the bottom of the compressor and plugged in a short ground wire I made that attaches via ring terminal to the body/chassis bolt that holds the trumpet bracket in place. The sound is just what I was looking for. A vintage Euro (Italian?) high pitched sound that speaks with authority but not in an obnoxious way. I might add that the air horns available now are plastic and not the metal horns originally supplied with the car. I don't know whether this will affect their longevity.
I hope the description and pictures help other Plus 2 owners who want to return to air horns original to the car.
Lee