air horn circuit diagram is it correct?

That earth to the right of the relay shouldn't be there... I was sure I erased it!  

If the air horn shown in the original diagram is the actual compressor and not a relay, then it does draw a lot of current, so that may be why the separate 20A fused circuit was added to supply the air horn. In any case, the control for the air horn has to come from the right side of the horn connections, like Hotwaterwizard shows, because the other side is grounded. Adding that diode in line with the RH horn will do nothing; though it might seem like the current flows that way and you could tap off the signal from the diode output, it doesn't work that way. Once it hits ground, it's ground. If the box shown for the air horn is supposed to just be a separate relay controlling the air horn compressor, then simply wire it as shown by Hotwaterwizard; if not, then just add a relay. No need for any diodes, because you're not dealing with any electronic drive circuitry and don't need to isolate anything.

Hi all, Thanks for all the replies! As above, I'm not very experienced with this stuff, so apologies if any of the below is crazy talk The airhorn box is an electric compressor (should have been clearer about that). The switch is to go in the cabin, so I figured the relay would be a good idea rather than wiring higher gauge cable all the way through to the cabin. I'm not sure if the existing horn wiring is high enough gauge to safely carry the compressor's current, so that's why I put a seperate 20A fused feed. Are either of those correct? Thanks again.

This should be all you have to do-https://img293.imageshack.us/img293/3449/aircompressorcntldx0.jpg diagram