You probably don't have any other pieces missing. Plunger first (if it is the proper length) then spring disc, then cup, then three boss plastic isolator. You must be sure that the bosses go thru cup and thru, and not on top of, the disc. Don't over tighten 3 screws. You are correct, the cup makes ground to complete the circuit to the relay thru the chassis. There is no hot wire to horn button.
