You're pretty much there, but its a 3 Ohm Resistor not 3 amp.

Maplins do 'em.
Only a Boss kit required, which demon tweeks can supply. It should have the horn wiring with it, so check with them if it doesn't.
Last thing, store the airbag with the momo badge facing upwards, so if any issues, it won't behave like a projectile and catapult itself through your garage roof or something like that.