Horn electrical conundrum

Just fitted a gps/alarm onto Skylark and all working apart from allowing the horn to signal when it's activated. There is one wire from the unit to connect to the horn which when tested puts out 12v when it needs the horn sounding, fair enough. Come to look at Skylark and find the horn is permanently supplied with 12v and the horn works by the horn push just grounding one of the wires (violet with white stripe). The question is how do I get the alarm to trigger the horn with a 12v supply or is having to add a second horn just for the alarm be the simplest solution?
Just Googling around looks like I need to use a relay but not sure how to wite that in my situation, I have a spare relay but a new horn just for the relay does not cost much.

Can't you add a relay? You already have 12V supply over there. Sounds the easiest solution to me. This way you don't need to change any wires and you can always go back to a original horn if needed.

sorry... just see that I haven't reed well enough.. forget my previous post

Thanks, you did read far enough, think I updated it while you were typing your reply.

I think this would work.. (just made a quick sketch)

Raggy wrote:Thanks, you did read far enough, think I updated it while you were typing your reply.

Haha, I was in a rush so wasn't surprised if it was me

Can I presume I have this right in my head, original on left, new with relay on right.
Horn sounds with alarm when alarm energises coil and earths the horn
Horn sounds with horn push as horn push completes the open circuit and grounds the horn
Always said amazed my wiring of the camper works, good job they were simpleton instructions!

Does the horn push not complete the circuit to earth?

Yes, noticed that after I posted it, think I got the diagram wrong slightly, I think the horn push should be on other side of relay in diagram then it makes more sense as I can short the wire to earth and horn sounds. Going to try it today anyway.

You can also do it like this..

Yes, thats a good idea!

Flipping good School Teacher you are Raggs. Not reading your homework properly.  I do it all the time.

Excuse me...at the moment ex school teacher Anyway forum to the rescue once again, worked just fine following PvDB's last suggestion. Thanks, I can now tell when I have turned the gps alarm on without having to txt it to find out! I can now hear it when I accidentally turn it on when keys squashed in my pocket which was more often than not.