Air horns, wiring is confusing!

Hi- this is my first post here so forgive me for my stupidity! I bought some air horns, pretty basic(!) and have managed to confuse myself as to what I'm doing wrong so I'll list the probs and if anyone can tell me where I'm going wrong I'd be grateful....The instructions are naff for them

1/ physical installation went fine...

2/the instructions have 2 diagrams- one refers to "switch connected to positive" the other to "switch connected to negative/earth".

The van is a Vauxhall Combo and I don't know what the difference is supposed to be? Can either be used?

The +ve diagram- indicates a +ve supply through the switch, then that goes through the relay and is earthed/grounded(see 3 about this too...) Could this +ve supply be a direct wire from the battery(a wire with no fuse going through the switch then relay?Is that well dodgy?) or "piggybacked" off another +ve wire?

The -ve diagram- this looks like a grounded wire passing through the switch and out the other side then through the relay where it is grounded again- after reading the relay tutorial I can't see where any power for working the relay could come from if the system is all negative?!

I can see the logic in the continuity for the +ve but in a nutshell where would the wires come from and all be connected?!

3/ where indicated that there is an earth/grounding (presumably through the chassis)can a wire be run instead direct to the negative terminal- such as from the relay and the negative terminal on the compressor? It's easier due to the proximity of the set up to the battery? Or is this a complete no-no?

my head hurts....

thanks for replying Mo but this is even less clear to me! Taking the relay as an electromagnetic switch there should only really be two circuits on this...On my diagram these seem to be

1 -CIRCUIT TO POWER HORN- +ve battery>Fused relay 30>87(when on, 87a when off)>+ve compressor >grounded -ve

2 - CIRCUIT TO TRIGGER/SWITCH RELAY-  +ve supply(what,where from?)>switch in>switch out>86>85>grounded -ve

again the principal problems are:

1/where I get the +ve supply for the switch, can I take a supply off the +ve supply to the cigarette lighter as it's close, or a wire from the battery +ve with a fuse to the switch?

2/can the relay and compressor groundings be wires fixed to the -ve terminal of the battery, or is it better to ground them to the chassis with self  tappers?!

I tried the with the grounding/earths for the relay and compressor to the -ve terminal on the battery, and the "30 from the relay" wire and one side of the switch(unfused, what should be the +ve) to the +ve terminal on the battery and it blew a 15a, then 20a fuse after briefly working both times. The recommended fuse rating for the compressor is stated at 16a and it's a fused relay I bought.

Electrics....the part of vehicle maintenance that gets me every time!

What is the advantage of air horns over other horns?

mo12v, So does the poster install a toggle switch inbetween the relay and the horns or at the end of the wiring or does it even matter? I am running into the same problem.

I have mounted air horns to my truck on the underside as I have plenty of room. Compressor has a pressure switch that auto shuts off @ 120psi. I want a toggle switch in the cab to select between OEM horn which I believe is (-) and Air horns which is (+) all to be activated by my horn switch in the steering wheel.

Right now I have a Single Pole 3 prong toggle switch that is also illuminated. When the switch is up the Air Horns should be armed and should activate when I hit my steering wheel horn pad.

-Power (I have it hooked up to the blue wire for horns on Fords [03 Ford Excursion 6.0PSD 4x4 Ltd.)

-Acc (I have this going to the Air Horns)

-Ground (Grounded to the frame)

I think that part of my problem is that my factory blue horn wire is (-). Any help would be greatly appreciated.

~WW

Actually it was two red wires (+) coming off of the solenoid and a (+) & (-) coming off of the compressor/pressure switch. All that needs to be grounded has been grounded so this is done.

I want my truck activate OEM horn and Air horn from the Steering Wheel Horn pad. I would like it to be wired so that factory OEM horn functions if steering wheel horn pad is pressed (only) if the toggle is off; and Factory & Air Horn if the toggle switch is on/armed and the steering wheel horn pad is pressed.

Will this make a difference in your suggestions?

My factory steering pad horn switch is a momentary switch correct? I am so new at this is is pitiful. Thank you for your time in my dilemma.

i install air horns in my shop. 

so that i can help you,

1.   tell me the make and model of your car and air horn. 

2.  tell me if you want the horn on the factory switch (steering wheel) or separate switch.

3.  do you have an air compressor and tank or is it electric.