I've never used a relay before so I i dont understand how to wire it up.
I'm assuming I have to use the relay because without it if i use the horn while the car is running the fuse blows. (i dont know why the car has to be running for it to blow but)

heres (http://www.stebel.it/stebel/NAUTILUS-COMPACT.pdf) a link to the wiring diagram

i kinda lost the 30A relay though, i have a 40A but. will that work?

im assuming we have a 'hot' lead, so i run the hot lead to 86.
run a power cable with a 20A fuse to 30.
then just earth 85 and run 87 to the horn.

if this is right, is there an easier way to power it then running a new power cable?

If you want your original horn and the air horn then there are a few ways to do it. one is to run a new power cable/relay/fuse, set ups as in that diagram (the 'instead of ones) only you'd install a button in the car and that takes the place of the steering wheel button as in the diagram. You can set that up using either hot or ground leads as if its an entirely new circuit. The other way is basically as shown in the diagram, you'd need a switch that has 3 connections. the idea is that you cut the direct wire to the horn and redirect it through that switch. That allows you to turn the horn's link to your steering wheel (via the relay) on or off so you can use both from the one button. Basically, the switch directs the power 2 ways which u can choose between. one will run the current the same path as it originally did, so the stock horn will sound. The second position links the button to the relay, which then will take power straight from the battery and hit the air horns.

The reason the fuse blows is because the new horn draws too much current for the existing circuit. A relay makes the current travel direct from the battery, rather than via the switch as all the switch does is trigger the relay. The car being on means the alternator creates a lot more current, so more available power obviously means the horn can draw more and hence the blown fuse. If you want to replace the old horn entirely then its a bit more simple, but you'll still need a relay to power it. But you're on the right track.

this site will be ultra useful for you too. http://www.rattlebars.com/mtz/hornrelay.html

with a 4-pin relay without a resistor,
power from battery goes into the 30,
wire from activation goes into 85, grounded from the 86
wire to the device goes from the 87

your 40A might have a resistor. a new relay doesnt cost much

thanks. worked it out. was easy as. waste of a thread at the end. oh well. somebody might find it useful later on.