can you wire one side of relay to the battery, other side (trigger) of relay to the old horn circuit.
trying to think of what else you would need to know/do
EDIT: I guess you should check amperage draw of the horn(s) and make sure your battery circuit can handle it. Then choose an appropriate inline fuse for the max amperage of your circuit, place it as close to the battery on the circuit as possible (positive side).
if you use OFC cable you can use smaller gauge wiring
https://www.knukonceptz.com/mobile-a...s-fleks-kable/
http://www.knukonceptz.com/default/a...cable_spec.jpg
EDIT2: can't help with mounting location sorry