Alarm Speaker(horn)/Car Horn HELP!

Don't think it's a good idea to splice the siren output wires into your horn. When the alarm triggers, it will back feed voltage into your horn circuit, and could damage it.

Most car alarms have a horn honk output (or wiring to support this). What kind of alarm?

Sirens are pretty water resistant. Best way to mount one is to face the opening down, so it won't collect water like a cup.

Alan

Put a small 5 amp fuse in series on the hot of the alarm wire(the wire going to the siren or horn) and hook it up and see what happens..if it dead shorts, it will pop the fuse and save your alarm or the electronics for the horn circuit..Thats what I would do..

there are a couple of obstacles to think about. the siren wire gives constant juice to the siren when its set off. do you want your horn to wail non stop for 60 seconds or however long when the alarm is set off?
most people use the parking light flash circuit, so that the horn honks intermittantly (like the parking lights flash). if you want to use the siren circuit, you would need a turn signal relay or something similar to break the horn honks up.

if continuing with the siren wire, you would also need a SPDT relay on the wire from the brain to the siren. then there is age old problem with the horn honk. most people like the horn to honk when the alarm is set off, but not when arming and disarming or giving warning chirps. different vehicles require different tools to ward this off. a lot of times it is trial and error or others' experiences with the same car and alarm.
if you try to power the horn with the alarm siren wire with no relay, you will kill the circuit. also use blocking diodes to keep juice from flowing backwards.

there are several ways to do it. i just mention this as something to think about. you dont want to try to do this and end up killing the old alarm siren or light flash circuit altogether.
good luck.

Try mounting the alarm siren inside the fender, behind the fender liner. That will get it out of the engine compartment.