If by the term "horn speaker" you mean an outside, or underhood speaker, I do not think it is wise to try
to run such a unit, directly from the radio. The radio has a remote speaker jack on the rear. it is best to connect a speaker in the cab to this jack, and tap off the plug to feed an amplifier, to feed an outside speaker. This is the way we connected radios through electronic sirens. the sirens have inputs that allow the low impedence of a speaker output to drive the amplified outside speaker. If you are using a non siren type amplifier, you may need to use a coupling transformer, to match the voice coil impedence, to it's input impedence. ~JC