Cannot get my horn to work on my "68 GMC

[gipper]

I have a nearly restored 1968 GMC Stepside. It had an aftermarket steering wheel in it which the horn was never installed. I found an original steering wheel and installed along with a new turn signal switch. I know that the switch is installed correctly. I cannot get the horn to work. If I put a jumper from the battery to the horn, it works. I have a brand new horn relay and the red wire at the horn relay has power. The black horn wire under the hood is not connected to anything, the green wire is plugged into the horn and the red wire runs to the positive post of the battery. I tried grounding the black wire under the hood and that did nothing. I need help!