The horn button came equipped in the following vehicles:
1987 Shelby Lancer (came on a 4 prong steering wheel)
1987 Shelby CSX (came on a 4-prong steering wheel)
1989 Shelby CSX (came on a 3-prong steering wheel)
1989 Shelby Dakota (came on a 3-prong steering wheel)
1990 Dodge Viper? (I believe a very short run of these cars came with the 3-prong steering wheel, for a short run of them came with the CS trunk badges that were seen on CSX's, Lancers, and the Dakota, too).
Unless you have either one of these steering wheels in your omni, OR a Grant steering wheel, then this horn button will not work for you. Here is a link to a picture of my Turismo, in which I have purchased the necessary adapter to install one of these steering wheels into an L-Body.
Just a heads up too, these steering wheels do not collapse. The cars that came with these steering wheels were equipped with collapsible steering columns. L-bodies do not have collapsible steering columns, but rather collapsible steering wheels. It is a risk putting this type of steering wheel in, but in my case, the Turismo is not a daily driver anymore anyways.
In addition, this steering wheel makes steering so much better! It is much smaller than the factory L-body steering wheels, and is also flat in design, rather than coming out towards the driver like teh L-body steering wheels. It is thicker in grip, however.
You will have to dremel about half an inch off of the back of the plastic covering that goes between the steering wheel itself and the dash board so that the cover will fit nicely around the top of the metal adapter plate. I went ahead and painted my steering column cover black so that it would match the steering wheel better and accent that portion of the dashboard a bit more.
Hope this helps! If you need anything else, PM me!
As far as it's worth, however, I would say, at most, in immaculate brand new condition, you could probably get $55 for it. I believe that is how much I paid for mine and it was in the wrapper.