On the gear shift.
dragonsaver got it right in one.
Bus#17 started life, like any other bus of its vintage, with the button for its horn mounted on the dash - you can see 4 buttons on the ledge beneath the speedo.
To sound the horn the driver has to lean forward and extend his right hand to make contact.
The company that runs Bus#17 has come up with a brilliant solution - they have taped the button to the front of the gear shift/lever. The driver can steer, change gear and sound the horn at the same time. And he can do all this without moving his body.
I would expect that the horn starts to toot automatically, when the ignition is switched on, and continues to toot randomly, regardless of traffic conditions!!
They may well be working right now on a plan to do this.
I guess it is possible that drivers of Bus#17 are not pressing the button to sound the horn - they only press it to stop the random tootling.
one of the foot pedels
If they put one on all 3 pedals - and they played different notes - it might make for an entertaining ride.