new horn; to relay or not to relay

Wanting to be able to give other road users a fair idea that I'm there I bought a twin air horn at the weekend and went about fitting it.  It came with a relay and some fairly straight forward instructions on fitting it.  Having not anticipated where I'd have to position it I bought what I thought would be enough wire to do the job but quickly found I was short and wanted to get the job done.

It seemed to make sense that if I wired in the little compressor to the same two wires that split into the existing horns then it would get power the same way they did.  This seemed to work in as much as the compressor made noise, but no noise from the horns, so to test the compressor was working properly I earthed the -ve on the compressor and touched the +ve to the battery and the horn rang out loud and clear.

So, would it seem likely that I can't just wire the horn into the existing electrics and that I have to use the relay they supplied?  Has anyone else fitted a new horn to their motor and how did it go?

thanks as always, jc

Hiya ive fitted these also to my mk1 and polo and they simply plug into the existing wires you allready have in place.
Make sure the compressor is mounted upright with wires at the bottom and if i remember you need to put oil in a little hole at the side of the commpressor and good luck. :wink:

Thanks for that!!  Think perhaps I was just a little impatient the first time, I've wired it in as you said and it works a treat  

thought I'd update this if anyone else feels the need to put a new horn in.  Whilst it worked wired in to the original wiring when the lights are on particularly and generally when there was any load on the electrical system it was quiet or didn't sound at all.

I ended up wiring it up with the relay and now its loud and clear all the time.  This might be perculiar to me, but there's my experience incase it helps anyone in the future.