Horn question 70 gmc sub

[67Mach1]

Got a question? I am about to get my sub inspected. Had it now for about 6 months, got plates for it in August. The death of my F150 is pressing the GMC into service.

SO now the question.. I seem to be missing a part ofmy wiring harness. I ahve traced the horn wire, green from the horn to the plug under the dash, ith has three wires in it, one green, one red and I think one black. It plugs into nothing. I then traced the horn wire from the colum into a black wiring "tree" for lack of better word. from there it goes? I can jump the wiring that is coming from the horn but would rather either plug it back in or find what is missing... (relay)

any ideas or experince with horn wiring, pointers or suggestions.

Thanks,
Thomas