wiring a aftermarket horn with relay

[joe 1950]

what i’m trying to do is wire up a aftermarket spartan horn with a relay in a 1935 ford pickup i’ve wired relays in other vehicles i no how they work but i can’t get the horn button to ground the relay truck has one original horn that works but on this system the voltage passes through the horn to energize the coil and is grounded through the button need. wiring diagram for the aftermarket witch didn’t get with it thanks for any help i can get