A lanyard would be a cord or Line above your head ? or side window I would think. This would open, operate a manual air Valve.
For a 12 volt
system, a Button, Becomes Hot, Live after you flip a switch. from electric horns to the air horn valve.
Maybe ? This is how it should be for either system.
Do you have an electric horn working Now ??? The switch may be in the , Town Mode
NOT the Air Horn Mode.
Try to trace the Air Line down to the air supply
to find a valve