Bill D - my apologies for wandering a bit off-track - that is a nice setup that you have done there, & decent electric horns are seriously loud, so I doubt you will have any trouble being heard !
Howard, the little blue gadget below the washer bottle is the compressor. The horn cct fires off a relay which feeds power to the compressor via 4mm cable, etc. The whole lot cost about A$55 a couple of years ago.
Kind Regards,
Peter D.