that black wire grounds the horn relay which sends fire to the horns. Ground it & see if horn(s) blow then see what parts are missing to make the horn ring on the steering wheel ground it(like you just did) to the round copper contact ring inside/under the steering wheel. Most of the black wire terminal ends are a flat 1/4" female spade conector. EDIT I just reread your post & that wire is(probably) not the(black) ground wire & DO NOT ground it! until you are sure which is which.