There is the wiring diagram:
http://67-72chevytrucks.com/vboard/s...27#post9186727
I highlighted the horn wiring in blue here.
I have these recommendations:
Find the wires I highlighted on the wiring diagram, and make sure it is set up like stock.
Test your horn relay to make sure it works. It is on the firewall inside the cab to the right of the fuse panel. If it is not working, get a fresh one from a 67-72 Chevy trucks parts vendor.
Test the horns by connecting them briefly to your battery with some jumper wires.
Get any Packard 56 terminals you need, and any replacement plastic terminal housings you need.
Make any spices to wires that you need with solder connections and heat shrink tubing, and if putting terminal connectors on the wires crimp and solder them.
If you need to unwrap and re-wrap any wire bundles you will notice they are wrapped in non-adhesive vinyl tape. That stuff is available from vendors and I recommend it. I got mine from amazon.com. It makes for a neat job.