I agree for testing the horns a relay is not needed, the relay is only used to reverse the polarity so to speek. the horn is grounded (-)through the case, the terminal of the horn must be connected to the Non grounded (+) side of the battery to work. The horn button only has a single wire going up the colume, so it can only ground out that wire to the colume, using the relay one side of the coil goes to the positive(+) the other terminal to the wire from the mast, making one circuit, the wire from the horn(s) goes to the third ternimal of the relay to get to the positive of the battery.
