Realize that the original horn electrical system did not use a relay and that your new harness probably expects to use one.
The original circuit had battery voltage going to one terminal on the horn. The other terminal had a wire that came back into the cabin and up through the steering column to the upper steering column bearing, which also provides a surface for the contact in the steering wheel to touch. The top end of the contact touches the underside of the horn switch in the steering wheel. When you mash the horn button the horn switch closes and completes the circuit to ground.
Check your new wiring diagram to see if it uses a relay. Making the change involves very little work.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
| 
