horn wire relay

I want to install and alarm with horn blast when the alarm is triggered, but I read I need to use a relay so the horn operates when the car is off. how do I wire the horn with a relay to work when the car is off

someone help

diagrams if possible

If your alarm provides a constant +12 to the siren (Most do) if you were to trigger the horn via a relay off of the siren, your horn would be constantly on (I dont know if this is what you want). If your car alarm does not provide  +12 to your siren when it is triggered, you wont be able to trigger your horn from the siren output (This type of alarm provides a signal or audio signal to the siren instead of a constant 12 volts). Use a multimeter to check this by triggering your alarm, and see if you get 12 volts.

For a solid blast (Siren must get 12 volts): Connect terminal 85 to ground, 86 to the + Siren output, 30 to ground or +12 volts (Depending on your vehicles horn trigger, either - or +) and 87 to the horn. When alarm is triggered, horn will steadliy blast the entire time the siren is on.

For repetitive honks: Connect terminal 85 to ground, and 86 to the parking lights output (Connect 85 to +12 volts if your alarm has a - parking light output signal), 30 to ground or +12 volts (Depending on your vehicles horn trigger, either - or +) and 87 to the horn. When alarm is triggered, horn will honk every time your parking lights flash (Like factory keyless entry panic modes). Note-If your alarm includes a remote engine starting system, dont do this, as when remote starters are activated, the parking lights come on, which would make your horn blast steadily. Also, since your parking light output of your alarm is tied into your vehicles parking light circuit, when you turn on your lights at night, the horn will blast, so you need to place a diode between the alarm parking light output and the vehicles parking light wire, so that a signal from the vehicles light switch will not be able to flow backwards and trigger the relay. For example, if your vehicle uses + parking light triggering, the Cathode side of the diode would be connected to the vehicles parking light wire. For negative triggering, the anode connects to the vehicles parking light wire.

Negative horn triggering repetitive horn honk diagrams