The part you need is called a horn contact bushing. The original GM part number is 12338143. Fortunately, the steering columns in the GM family, as well as many other OEMs, were manufactured by Saginaw, so many parts are interchangeable between vehicle lines from year to year and model to model. The color of the bushing your truck requires can vary from green to clear to white (see attached image). Prior to 1973, there were dimensional differences in the bushing. New old stock (NOS) and suitable substitutions are still available through Internet searches.
The plastic horn contact bushing (#3 in the schematic drawing) is an electrical insulator that allows the upper horn contact assembly (steel Belleville spring plate #2 and steel cup #4) to "float" between the plastic horn button cap (#6) and the steering wheel (#1). When the horn button cap is depressed the Belleville spring distorts and contacts the bare steel hub of the steering wheel to complete the horn relay circuit to ground, sounding the horn.
The schematic shows the order of assembly incorrectly; the order of the bushing (#3) and cup (#4) must be reversed. That is, the Belleville spring (#2) is installed convex up against a plastic seat encircling the steering wheel hub, and the three towers of the plastic bushing (#3) drop through the three holes of the cup (#4) and Belleville spring (#3) from above the cup so that the spring resides undistorted and the assembly is allowed to float while remaining electrically insulated from ground.
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