Two horn conversion question.

Your '74 may have the horns mounted under the battery tray instead of on the radiator yoke (my '75 does, I believe they moved there when a radiator overflow bottle was added in '73?), but the wiring for both horns should still be in place- you just need the late brackets.
Okay, I just checked it out under the hood. My Duster's horn power supply is a single green wire that has a female spade connector at the end of it. There is no jumper from that connector to a second female spade connector to operate another horn. Does this mean I need to modify my wiring harness to include a jumper to a second horn? My horn's mounting bracket horn is held onto a battery tray support bracket at the rear of the tray with a single bolt. Do you think that I need both of the horns off a car from the junkyard, or can I use my existing horn and simply add another. They need to be two different tones, obviously. Finally, does anyone know where the second horn mounting bracket goes? Photos would be great - thanks!