VW Electrics Adding an independent or new horn
Covkid: Some may be looking to see how to add an extra horn such as air horns or a klaxon horn. If you want to use existing horn push, read further down but understand the principles by reading all of this first:
Independent Horn: To add an extra or alternative horn, get a 30amp relay (minimum), wire pins 85 & 86 to earth on one, and a 12v feed with switch on the other. You can mount switch in dash.That gives the relay the trigger to switch current for air horns. At this point, if you press button, relay will click.
Now for power to horns. Remaining two terminals, 30 and 87 - you just take another 12v feed using slightly heavier cable from battery to one terminal (with a fuse) and the other terminal goes to air horns. Applies to any horn that draws more than a bicycle bell. As long as you group 85 & 86 for trigger and 30 & 87 for horn supply it'll work, even if each pair is reversed. You need the relay to stop horn button burning out.
In this instance I mounted the button more over to passenger side (though still in reach) so my youngest could press it when he sees other VWs. Maplins sell these large red 'push to make' buttons for under £3. The relay and heavier gauge wire for horn itself is essential to stop current-hungry horns heating up the switch or wires. This circuit applies to anything that draws current with the relay and associated fuse matching whatever you're fitting.
Horn Upgrade: If you want to use the exising horn circuit, understand that it is a constantly live circuit all the way up the steering column and the horn botton merely grounds it to make the connection. As it is, this circuit won't be sufficient to drive anything beyond the original horn which includes most aftermarket horns without burning switch contacts, so you need to find a convenient point along the existing horn push wire (at fusebox is fine) and connect the relay into that using pins 85 & 86, as in schematic above.
The remaining pins (30 & 87) create the secondary circuit to power your new horn. Just remember to fit something like an 8 amp fuse for the secondary circuit to make sure its protected.