My car had airhorns fitted by the previous owner but in her haste to remove some of her other electronic gizmos she pulled the red wire out for the horn. There is a loose red wire for the horns down by the drivers feet under the steering wheel but I don't know where the horn wire goes. Does anyone have a wiring diagram or a picture of where I should plug the wire into?

 

Does your stock horns work? That red wire may simply be the old wiring for a relay for the air horns. It may not go anywhere. Check it for voltage when you push the horn button. You may find that it's just dead and you can just tape off the end and ignore it.

 

When I got the car no horns worked because the little stock horn was disconnected. I reconnected it and the stock horn now works but the airhorns don't. On this basis I figured that the red wire must 'go' somewhere to activate the air horns.

 

You can't assume that the wire you found was meant for the air horns or even to know exactly how they were wired. You're going to have to start at the air horn compressor and trace all the wires backwards and see where they end up. Just plugging in a loose wire you find and hoping it does the thing you want is dangerous and foolish.
You don't know if that wire is actually for the horns. You don't know if it's a wire for a switch or a wire for a relay or whether it goes to or comes from a switch or relay.

 

Sorry, I probably should have said but I have already traced that red wire from the airhorns though the bulkhead to down by the drivers feet. I am therefore quietly confident it just needs plugged into the module with all the wires going into it (above the drivers feet). It only became disconnected from that location when the previous owner hastily removed her lazer jamming/speed camera device.

 

Not sure what this "module" is, but if you think you've got an understanding of it, then plug away. Without seeing the whole wiring package, it's impossible for anyone here to tell you what the result will be.