The housing around the shaft is hollow and that's where the turn signal wires, etc get fished down through. However, steering wheels turn, whereas the turn signals don't. Chances are there is a clock spring underneath the steering wheel hub. This clock spring contains fine wires that will be used to pass the horn signals down from to rotating wheel to the stationary column.
On my Freightliner chassis I have a SmartWheel, with lots of buttons on the wheel. These buttons squawk codes down to a SmartWheel module, which then operates a bunch of relay circuits to control wipers, lights, horn, etc. My clock spring has 4 wires. Two to send the codes and two for backlighting the buttons on the SmartWheel. Clock springs are also commonly used in cars and trucks for the same purpose.
If your coach uses a clock spring, it may have 4 wires in it. Two wires would be used for the horn but you may have two extra wires that are unused. If this is the case you could utilize those two spare wires to control your switch. You could probably share a hot feed with the horn if you needed that.
That way you can control your Photon Torpedo Launcher right from the wheel rather than having your Tactical Officer clear the lane ahead for you.