You have some confusion about electrical theory. The relay does not REQUIRE 15 amps. That means the contacts can handle up to 15 amps without overheating. Switches do not require voltage, (electrical pressure). They are typically rated something like "10 amps at 12 volts". That means its contacts can handle 10 amps when it is turned on, and when it turns off, it can handle the resulting arc created by a 12 volt circuit. I'm not doing a good job of describing that, but the switch has to handle the very tiny current the relay needs to turn on.
We use relays because they use a very small current to switch a large current on and off. Without a relay you would need a giant switch. In a horn circuit a 10 amp switch is serious overkill. A one amp or two amp switch would be more than enough. A pair of horns usually draw about ten amps or less so you wouldn't even need a relay if you used a 10 amp switch.
As far as how to wire it, you can look in any service manual for an older car. Newer ones from the mid '90s involve a complicated and unreliable computer. Almost all older cars used a three-terminal horn relay. All relays have at least four terminals. In the horn relays two functions were done by one terminal. If you have the common four-terminal relay, two terminals can be tied together. If you have the equally common five-terminal relay, one is not used.
Thursday, May 30th, 2013 AT 9:59 PM