First of all, nice avatar.
Moving on, it depends on your valve. If you get a manual valve, just run a rod/ cable into the cab to pull or push to work it. If you get a solenoid valve, then it will need a hot and ground wire, with a switch. You can run a wire from battery hot to a switch, then from the switch to the valve, then a ground from the other post on the valve to the frame. Or, you could run a wire from battery positive to the valve, then a wire from the ground/ negative post on the valve to a switch, then from the switch to a ground on the frame.
I'm not sure how the SD factory horn works. On my OBS, it was a hot side switch that grounded through the horn assembly. I basically removed the hot wire from my factory horn and ran it to the solenoid valve, then a ground off the other side of the valve so that my factory horn pad triggered the air horn.