While the bumper was off, the horn is very accessible. Since the volvo horns don't have a grounded body I have to use two wires and ground one side of the horns. The volvo horns came from a '86 model and I got it from the auto recyclers. I cleaned it up and painted it blue. One horn has a higher tone than the other which makes them sound good.
The original single horn has only one wire because the body is grounded. I also have a "bad boy" air horn which is already installed but not wired yet. I'm just waiting for the weather to warm up a bit before I wire it up. I will install a relay and switch to select which horn I want to use.