This is pretty basic wiring and should be straight forward
even if you are starting from scratch. You can use 14awg wire throughout.
Relay - The advent of Air Bags has made the real estate in the steering wheel more valuable. Consequently, Horn switches are small and light duty. Most modern vehicles already have a relay to route the horn current (as high as 150 watts) around the switch. Hella includes a relay with every horn kit, but most likely, if your car has an air bag, you won't need to use it.
Grounding - If you are sure
you have a good ground, you can ground the horn(s) locally instead of running the wire
back to the battery. Many cars (Subarus for sure, probably others) ground their stock horns through the bracket. In these cases you will only have one wire running to the stock horns. You will have to run a wire from the second terminal on the horn to the mounting bolt, or some other ground.
Polarity - Diaphram Horns do not have a polaity. Air Horns must be wired with the correct polarity, or the compressor will run backword and it will suck instead of blow. Sucking air through a reed makes no noise.
Switched Ground – The control circuit assumes
a conventionally wired OE horn system. That is, the +12V is switched to the horns. Note
that in some Japanese cars, the GROUND is switched instead. In that case, you would apply
battery to pin 85 of the relay and the horn circuit to the other side of the on/off switch
or directly to pin 86 or the relay if you choose not to use the switch.
