On my S150, the horn is at the front where you step in. Its also activated by the trigger on the left stick. Lif the bucket up and get someone to look to see if there is a horn installed in the hole. If its not installed, it should have the wiring there to install one.
The hole in the back is for the backup up alarm. As has been mentioned, the harness is standard equipment, it doesn't mean there ever was an allarm attached, If you remove your dash, you will see where you can add options for power bobtach etc, but the switch is not installed, nor is the valves to operate it, but the connectors are still there.
If you want to install a horn at the back, why not get a standard horn and mount it backwards a little so its protected. The noise will still get out of the engine bay, they are pretty loud, people will surely hear it!
The OEM backup alarm is bolted to the back door, you will see two holes under the hole, this is where it is secured. Buy genuine, or simply make a bracket, cheap yet effective! You will need to add wires to operate it though, as this is not the standard horn wiring back there.